Hi kirby. My plants are mostly grown in fairly low nutrient hard clay pond bottom. By hard I mean that there isn't a thick layer of slime that you sink into when you walk on the bottom and I never sink in farther than my ankles. I'm not saying that waterlilies like hard packed low nutrient clay I'm just pointing out that lack of good soil, as long as the leaves are nice and green and not yellowish is probably not your problem. Two things that I know that will cause lack of flowers are over crowding and lack of light. I guess that your problem is overcrowding because that will cause small leaves more than shade will. The answer is more room per crown and as much direct sunlight as possible. Assuming that you have lots of sun and don't have too many crowns in each pot the next best thing I can think to do is to plant your crowns in much bigger especially wider pots, and I mean BIG pots, WITH holes in the bottom. If you have fish that like to dig at the holes then stack a few rocks around the pots. The roots will grow through the holes and the plants will think they have even more room. Loose soil is just fine as long as it's heavy soil, low fertility is best. your plants should be causing a lowering of your waters fertility,not an increase. If light is a large part of the problem and you just cant get more then the variety you have becomes more important. some varieties do much better than others in low light conditions, I hear, since don't have any low light ponds I cant help much with that without getting a book out. Your growing season is probably not a problem ,and if it is ,choosing the right variety is the answer. And again someone else could probably help you more than me. Just ask and I figure a bunch of people from up north will suggest there favorites. I fertilize only when my leaves start to lose there nice green color. 2 or three lily tabs per crown . Mike.
