I don't know your clutch, but many have an adjustable air gap. You may have such a clutch. First you have to find it and then look for 3 adjusting nuts around the perimeter and an oval slot between them. That would be where you put in the feeler gauge. Around .012 gap is usually safe. Tighten the nuts very slightly, evenly, to get that gap at all three points, with the switch OFF.

If there is nothing causing any "extra" resistance I really have to say "ditto" to allowing a minute (or two) to let the engine warm before applying the PTO. I have read on this forum that it is actually recommended that the engine be running about 1/2 throttle before engaging the PTO (for mower deck). If you have been able to engage PTO at lower RPM in previous years without stalling the engine, you may just be seeing the cumulative age of the machine beginning to show (and don't forget today's gasoline may not be a power popper in all machines). Another detail is.......don't engage the PTO when you are "in the grass, with deck down". If you are already in the grass and have the deck down when you engage the PTO, the blades will have a harder time getting into motion and up to cutting speed. Engage the PTO first, then lower the deck.

Another facet of operation when an electric PTO is present: Your electrical system had better be charging correctly and the battery be in good condition. Low system voltage can make the PTO reluctant to engage, especially if the air gap has grown too large. Low system voltage might occur if you try to engage the PTO at low engine speeds. If you crank the engine, then try to engage the PTO immediately, at low engine RPM........the system might have low voltage. The battery may have gotten a bit discharged since you last used the machine, then you operate the starter to crank the engine (more discharge), then you run the engine at low RPM where the alternator output is weak............at this point, the system voltage can be too weak for dependable engagement of the PTO clutch. Not saying this is your problem, just saying that sometimes "a result" (non engagement PTO), can have more than one specific cause. Even if you have read this before, it never hurts to have a refresher.

Do you know if the mowing system on this tractor has a "blade brake" built into the PTO clutch? The blades must stop within about 3 seconds (federal mandate) after the blade drive is disengaged. Some OEMs build the blade brake into the deck pulleys configuration while other OEMs are using a PTO clutch that has the blade brake function built into it. If your PTO clutch has a blade brake built into it, there are a couple of things I can think of that "might be" causing the symptoms you are seeing. 1, the PTO clutch is defective. 2, the PTO switch (or other items in the circuit) has become worn to the point that the internal contacts are "chattering" (for want of a better term) and so the clutch is attempting to disengage and re-engage rapidly (chirping?) The fact that you are having trouble getting the PTO to engage now could be pointing to either a defective PTO clutch, or a defective PTO switch. Beware that the second scenario could be caused by ANYTHING in the PTO clutch circuit that has (or makes) a poor connection between the PTO switch and the clutch itself. This means that a loose fitting wiring connector (maybe at the clutch itself) anywhere in the wiring harness might cause the kind of electrical "chatter" I described. Some PTO circuits employ the PTO switch itself to handle the electrical load of the PTO clutch. Some other machines use the PTO switch only to operate a relay that carries the actual current load to the clutch. If a relay is used, scenario #2 could also be caused by a loose connection in the relay or related circuitry. Eliminating any switches or relays to isolate the problem could require hooking a temporary jumper wire connected to the PTO clutch wire port on the clutch (after unplugging the OEM wire). If one were to opt for this as a test it would be necessary connect the wire to the clutch first and then connect the other end to a battery positive point after starting the engine. If a 12VDC power plug is featured on the dash board, that would be an ideal place to connect the top end of the jumper wire. If no 12VDC power plug is featured, I suggest that a jumper wire be made up that includes a 20 Amp fuse along with a simple single pole toggle switch to be connected to the battery + post. Tape any exposed terminals on the switch. In lieu of an actual toggle switch a simple "male/female" wire connector pair could be used to connect the jumper wire together for the test, though it would a bit more cumbersome to use than a flip switch. Before I resorted to doing the jumper wire test I would certainly do a thorough visual inspection of every wire connector from the PTO switch all the way to the PTO clutch to be sure that all those are secure and free of any corrosion and that there are not any signs of burning/melting of the metal terminals or the connector body.