First: Check in the under-side for a stick that might have gotten stuck in the mechanisms. Then look for grass build-up in the pulley grooves. Also, inspect for a broken control rod, or anything that might limit the correct operation of your machine.
These are some of the things i have found, over the years, of why a belt won't tighten up, even when bought from the supplier.
Also, a hardware store-fits anything-belt may have a different shape than the one required for your machine.
(Yes, i know, the last one you bought there worked, but--maybe the angle of the belt sides was changed by the MFGR. of the belt! Thats why it is best to buy the belt from a legitimate lawn equipment parts seller, who will have direct contact with the maker of your machine!

You called yourself a "NOOOOB". I gather from the emphasis (OOOO) that you might have routed the replacement belt wrong, and thus the tensioning idler pulley is not able to do its job.
Did you find a "belt routing diagram" decal on the deck
that shows the correct routing.
If you have not seen the decal, it may be missing.
Post the full Toro model number and the serial number (get both numbers from the chassis data decal, usually under the seat or hood).
With those numbers, perhaps we can get online and locate a belt diagram to make sure the belt is routed correctly.

ya the belt came from the manufacturer, i cleaned the mower unit up pretty good, no big sticks or debris, just some bits of grass that aren't in the way of pulleys or the idler arm.

i did put the tension cord attached to the engager lever and adjusted that backwards which seemed to help tighten it up a bit but i was only able to get maybe a centimeter worth of play from that and it doesn't seem to be enough. it works better at max height but still slows right down to almost a stop when lowered even one setting.

when working on the mower unit when stopped i can easily turn the blades from underneath the unit and see the belt slipping on the pulley of the blade im manually turning even when the tension level is engaged. could this be an issue?

i'm wondering if my blades being dull would attribute to this at all?

I have experienced that problem on several mowing decks.
One was an old Dynamark lawn tractor. If/when i installed the belt, it was too long! Even though i had bought it from my faithful supplier. Took it back-they said it is me, not them. So, i rechecked it and found if i routed it one way, it was correct, and the other way made it too long. I gave the machine to my son, and he complained about it being the wrong belt--not so--didn't do it right!
Then, on a newer Simplicity Broadmoor, i routed the deck belt the wrong way, and it was way too long! Finally realised i was wrong-rerouted it and it fit well!

i will take another look at the diagram but from what i can see i't on 100% correct, i will take it right off and redo it again.

thanks for the tips, keep them coming please! i'm not sure if this will solve the issue

***"thanks for the tips, keep them coming please"***
Well, you haven't posted those important numbers yet, so we don't have a very good idea of what you are even looking at.

Typically, when you purchase the OEM belt, you don't even have to adjust a thing, unless of course, you had previously adjusted tension to compensate for wear on the old belt. In that case, your new belt would be too tight and you would be seeking advice on how to loosen the belt some (or at least, MAYBE you would).

I am surprised that rustyj did not mention his good advice on how to tell quickly if a belt is put on wrong.
That advice is: The VEE side of a belt must fit into a VEE groove pulley............the FLAT side of a belt must fit into a FLAT groove pulley.
So, a quick look at that bit of detail might at least tell you if you have erred, or not.

sorry been away for most of the day, here's all the info on the machine and the belt i purchased

Toro 14-38 HXL (71217)
Model 28N777
Type 1174E1
Code 870505ZD

Belt Part # 88-6250 (manufacturer belt)
87" long - 1/2" wide

i double checked and even re-installed the belt and having the same issues. belt is DEFINETLY on correctly after checking/triple checking the diagram.

back story: the first belt was way to small as my repair man put it on not thinking about the tension level and just ended up using a piece of string to get the measurement. that belt broke after one use while i was going thru slightly wet and tall grass, i started to smoke ALOT got off after abour 30 seconds and only half the lawn finished. Re-mounted the machine and started it up after about 5 minutes and right away the belt snapped. the original belt that i had would easily have allowed me to cut that type of grass.

Do you have all the springs as shown? and in their proper places?
Pay close attention to the positions where the springs hook to the idler pulley arm (item 32).
If someone reversed the positions of the 2 springs, it would cause the weaker engagement spring to lose power while the stronger extension spring would gain power.
Of course, this presumes that the 2 springs could actually fit in both holes in the arm. But, it's a thought to check on.

{{gwi:341635}}

The lower end of your cable is seriously adjustable. Does the engagement lever itself feel secure in the tower?

both springs are present and haven't been switched since i've owned it.

i've already adjusted the cable as far as it will go, anymore and it wont thread.

"Does the engagement lever itself feel secure in the tower?" There was a reason that question was asked. I'm referring to the engagement arm and how it is supported. On a 97 model, I seem to rememeber still had each end of the rod mounted on a rubber block rather than mounted through the plastic tower w/ a nut on each side. The new style tower i think got rid of the tilt steering feature too. The rubber mounted ones would pull out if they had too much tension on them- IE from the too small belt previously installed. By this time you also have notable wear in the arm and fastener on the top of the deck. That may contribute some as well. If you remove one of the fuel tank bracket nuts and swing it out of the way- the fuel tank will lift up so you can have a look at the control and cable. If the control rod slipped off of the tower, call 18003482424 and see if they will send you an updated tower.

the engagement arm definetly has a bit of fight in it when trying to get it to go into place. nothing as intense as if it felt it weas going to break but enough that seems like it should be normal.

and ya i guess it does feel secure in its tower, there is no side to side wobble and it doesn't feel like it's not doing naything.

i will try and pull the gas tank off and take a look at the tower.

i definetly have a 95/96 model as i check on the toro site.

I am having the same exact issue. Did you ever get to the bottom of this issue? I dropped by my toro dealer this morning and he said it could be a worn pulley on the motor causing the belt to sit too deep in the pulley. Even a mm of diameter change could add up to an inch or so... I have ordered a new pulley and tensioner bushing. Will see if this fixes it. Let me know how you made out. I can cancel my order if it is something I didn't think of.

Did you ever figure out what was causing the loose belt and were you able to fix it?

I would really like to know the end. I am having same problem as OP.

It turns out both the belts mentioned in my earlier post are wrong for this mower. One is too loose, the other too tight. Find a belt that is 86 inches long.