I have a blue giant hosta. It is in fertile soil and gets plenty of water. I had been dividing it every few years but lately it has started to shrink. I don't want to lose it. How can I get it back?
I have some hostas myself, growing in partial shade; I water them whenever there is a long, dry spell and also feed them (they are in a light, sandy soil which is very free-draining and soon loses nutrients). Every so often, I divide the clumps in spring and, apart from some slug damage, they are all thriving, so, unfortunately, the advice below is not based on personal experience; however, judging from what I have read, I think the problem is likely to be caused by one of more of the following:
There is some useful advice on hosta cultivation by the Royal Horticultural Society here.
Further to your comment below, the conditions you describe seem ideal for a hosta; and as it is not showing any signs of disease, the problem could be that your soil has become too acid, which often happens when soil is fertilized and manured for many years. It is worth doing a pH test using a pH meter (the ideal pH for hostas is 6.8-7.3) and, if your soil does prove too acid, you could lime it.