Granola Gardener had a similar question, but your symptoms sound more severe. Take a look at my reply to Granola (I linked it and also pasted it at the end of this message)...

But in addition, it really sounds like your plants are root bound. Gently lift them from their pots to see what the soil and roots look like. You may need to pot them into a larger pot (see my notes on potting soil for hibiscus below) and/or prune the roots a bit.

Also, in the fall go ahead and prune the plants, very drastically if you like. Make sure the roots are ok in the pot and that you haven't been overfeeding or underfeeding. Hibiscus generally bloom on new wood (especially tropical) so pruning before winter not only helps to overwinter but also gets new wood growing by spring. Since you're in Winnipeg, put them in the brightest room you have, and maybe supplement with additional plant lights.

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Here's what I wrote to Granola:

Let 'em dry out a bit (just a bit). Hibiscus don't like "wet feet".

Also, many so-called hibiscus fertilizers are horrible for hibiscus (way too high on the phosphate). Try a tomato fertilizer or dig some Osmocote into the soil. A low middle number (7-2-7 or so) is a pretty good route.

Also, how long have these plants been in the same pots? They may be root bound, in which case you can root prune (trim a bit of the root ball all the way around, and do normal root bound procedures to encourage new roots) or simply pot up. For hibiscus potting soil, I mix regular potting soil with perlite, bark, and some cactus soil if I can find it -- also a little potting charcoal if I've got it. That makes a very well draining soil.

Finally, since you say you keep them moist, you may have salts built up in your pots from the tapwater. Let the soil dry out and then run the hose on the pot with a gentle stream til a good amount of water runs through the bottom. This should help wash any salts out.

Here is a link that might be useful: Granola Gardener's hibiscus question

What do the blooms look like when they did open? Also, what shape are the leaves.

I purchased some hardy swamp hibiscus for a friend. Hers have red stems which grow to heights of about 8 feet tall. the leaves are shades of bergundy and green. Those leaves also have the deeply lobed shape of some oak trees' leaf. The flowers are a beautiful red and stay only 4 to 5 inches in diameter, so they are not too heavy for the canes to naturally support them. Sorry that I forget the cultivar name of them and if they seed or not.

If you have a hardy hibiscus growing in your pots then they need plently of moisture. The hardy swamp hibiscus will actually begin to droop and develop some yellow leaves if they are not getting enough water.

This is a tree about 4 feet tall. Its some kind of hybrid that was new to our garden center this year. The flowers were a beautiful peach color with a double bloom on each flower. I think the problem may be to much water, I have been letting it dry out for a while now, but it has lost most of its leaves now and almost no blooms anymore. I may pull it out of the pot today and take a closer look at the roots .

now I am confussed is it too much water or not enough? Is osomocte enough or should I feed with miracle grow every other week?

I have two Hibiscus plants in raised beds in the two corners of my back yard. I have little control over the moisture, since it rains a lot in Houston and I don't water them unless they get really dry.

I cut them back in the fall and they grew like crazy this spring. This first set of flowers was fine, but after that the buds just turn brown and fall off (in both beds).
The garden store I purchased them from said to fertilize them every other week, with a high nitrogen fertilizer, which I have done.

The ground her is just some top soil (18 inches above the grass where the plants are) and underneath it is just clay, so no drainage other than that due to the raised beds.

This is their third year and they bloomed fine the previous two years and I also fertilized them less.

Suggestions?

Dave, I'm baffled..are your hibs tropical or hardy? If they're hardy, are you going to plant in the ground?
I grow both types..tropical are kept in pots, summered outdoors, fertilized w/hib fertilizer once every 3 months. My hibs have done well since I started using hib food..I had killed many in the past. My oldest tropical is 14 yrs this summer. Pot should be large enough to hold roots w/about 1" per side for extra root growth. I allow soil to dry between waterings..they don't like wet feet. They're inspected once a wk for insects. Misted daily.
Hardy hibs and Rose of Sharon are fed twice a yr. I don't prune..actually, my larger ROS is pruned to a degree, mainly to be kept a certin height, but I let it go and it outgrew me by 4-5', now I can't reach the top..lol..Started this tree from a cutting in the early 90's..
If soil is leached, there shouldn't be any salt build up from fertilizers, so for those who fertilize, try leaching once a month or so.

Tropical hibs can inquire chlorsis (iron deficiency) It's a good idea to add iron 2-3 times a yr.
In winter, place in the brightest window possible, in a semi-cool room. For those in cold, gray climates, feeding should be stopped..Unless plant is still flowering.
Once heat is turned on, I mist..it helps w/humidity and keeps leaves clean..smaller hibs can be placed in sink and hosed. Taller, goes in shower.
Hibs are placed in south windows w/additional light at night. Once plant goes dormant, I withhold fertilizer.
Anyway Dave, what type of hib do u have? LOL Toni

Hi I am writing from India. I got a hibiscus plant in a pot from the nursery last week. It had buds and the plant was healthy. After it came home I put an ounce of organic fertilizer and watered it. Within three days the biggest bud and associated leaf turned yellow and fall off. It has got new leaves though. I water it every morning. The temperature here is like 42 degree celcious right now. Moreover, new buds and leaves have been drying and falling off in other plants also. I give proper water, fertilizer and even insecticidal spray often but no help. I notice small spiders moving around the plants. What to do?

Hi Jyoti. Hello from Canada :) I too am new to the hibiscus world and was so distressed by what happened to my plant in the first few days that I looked everywhere for an explanation. I found myself on this site asking for answers and see we have a really similar issue. Although, 42C is so super hot (we don't get temps like that in the northeast!) and my plant doesn't show any signs of bugs, I feel for your bloom and bud drop dilemma. PhotoSynthesis provided some much-needed relief so you can always take a peek at his info. Hopefully others might have helpful advice concerning this and at the very least, be sure to keep us posted on your little plant's progress!