They are probably getting overcrowded in the pots and need to be divided this fall after the stems and leaves go brown. You will likely have lot's of extra bulbs to either pot up or plant in the ground. Also have you fed them? They may have depleted the nutrients in the potting mix. (Don't feed when they're blooming, it can lead to bulb rot.) Are you in a place where you are unable to plant them in the ground? Though lilies can be grown in pots(IMHO) they are much higher maintenance in pots that if they were in the ground.

thanks for the reply! so, what you are saying is that overcrowding and lack of feeding have created this problem? sounds about right to me. i shall try to plant them properly in the ground (my original intention, but i got lazy). i think my thought in potting them was to keep the voles from eating them, as has happened before when i planted in the garden beds. even with caging the bulbs, somehow i kept losing them instead of having them increase into nice large patches.
i love lilies, but just can't seem to grow them well... ah, well, onward and hopefully upward! thanks again for the help. =)

If you have vole problems, that's another story. There is some type of solution you can dip the bulbs in which is supposed to repel critters from eating them. Unfortunately, at the moment the name is not coming to mind. Maybe someone else knows what it's called.