Lilies don't require alot of fertilizer as a rule.
What kind of soil do you have....acid or alkaline? Do you get alot of rain?
I would try digging them this fall and see it you are missing some bulbs....could something be eating them from below?
Flora

Tons of rain except in July and August, acid clay soil that has been amended and top dressed with compost for fifteen years.

There are commercial lily hybridizers/bulb growers here in the Willamette Valley up the ying yang. I know Lilies grow well here.

Don't think I am missing any bulbs. They are coming up just not as vigorous as they were before. Weak and spindly.

I have outdoor cats and any moles that dare venture into our yard are quickly killed, usually only one or two hills a year. I don't think they get a chance to create tunnel systems that voles could be using. Squirrels stay up on the fences or in the trees.

I am in Zone 5 (Canada) and noticed the same thing. I bought some Lovely Girl last year, and one stem has grown and just at bud time it started browning from the bottom. The other two stems have large buds, but the first one took about 3 days to open, and is still having difficulty looking really spectacular.The third stem has buds, yet to open, but it too has leaves yellowing from the bottom. We have had lots of rain here (record for the month of July) and I wonder if perhaps there is some bulb rot taking place. My Stargazers have declined to only one this year which is only about 12 inches tall with one bud. This is down from about 5 which used to grow to 24 inches and have multiple blooms. Perhaps the hybrids are being taken over by the species lilies.

I've had the same problem here in MD that you both describe. I found something either on this forum or on the cutting garden or cottage garden forum.For 3 years everyone was saying the yellowing etc. was due to a fungus called botyus or something like that and that it would come back the next year. Well,for me it never does, so I explored what I mentioned above and there is another fungus among us. I am spraying a few days a week, also liming as it is said that fungus doesn't like alkaline soil. Oh, just found the fungus name: fusarium oxysporum. Put that in the search here and you should see folks complaining. Its related to the tomato blight etc.
Like you, my bulbs are not triving and getting more buds each year (except for 1, my favorite silk road) Even some of the ones I purchased last fall are
spindly and like you, they die from the bottom up.I had a white lily that had 4 flowers on the very top, and no leaves under it. Now that we have rain, I am going to dig up one of them and see how the bulb looks.