If I were you I would probably dig them up this fall, clean off the dirt, locate all the bulbs, and see if you can see anything the matter with them. I had one variety that was really struggling and kind of sickly each year, so I dug up the bulbs and they were discolored and looked like they either had some disease or rot.

Could definitely be rodents, but you should be able to see evidence of them, either piles of soft dirt on the surface somewhere in the area, or else small holes somewhere. Could be overcrowding...I haven't had any lilies for more than 5 years and they keep spreading...but I don't think that lilies are supposed to fizzle like tulips sometimes do.

Anyway, after you check the bulbs, I would divide and replant and see if that helps. If they are already declining, dividing them and seeing what is going on certainly can't hurt. I would probably wait until fall though, otherwise they may come back even weaker next year if the stems and roots are disturbed during the growing season.

Just what I would do!

Oh for sure, 5-10 yrs, yes, its time to split them up in the fall, once you see that they are not as vigorous and the flowers are small, that is a sure sign that they are crowded. Top dress with compost or bonemeal in the fall won't hurt either.

Sharon