Disappearing Covers

I have over 2000 books and I have been very careful to make sure (even if I had to upload) that I have cover art for each one. It is possible I missed one or two over the years but a few weeks ago I noticed many covers missing. I fixed those and went through my whole library fixing all I could find. If I missed any it wasn't much. Now many are gone again. Not necessarily the same ones as before (can't really remember for sure).

Anyone know why this is happening?

Ok...sorry I see someone asked about this below. I guess I mostly use Amazon covers and this is the problem.

For any future reader's easy reference:

What's going on with covers?

If you're using Amazon covers, then you are not in fact making sure you have cover art for each one. You're making sure that, at that particular moment in time, Amazon has it.

Amazon can change covers without LibraryThing even knowing about it. I strongly advise people to upload their covers to LT. You can even do so by grabbing the web address of the Amazon picture. Look under "change cover" in the left hand column of your book page.