Missing Covers in some files

I am desperate. I have a lot of mp3 audiobooks and I want to embed the cover of each one.

Sometimes it works, sometimes it doesn't. I can't see where the differences are at all.

I tried to find missing covers or tags without a cover with the appropriate filters, but it doesn't show me any faulty ones.

Can anyone help me further?

What did you do to filter?
What file format are the audiobooks? (flac, mp4, mp3, wav?)
Have you checked the files for integrity? (No, it's not enough if they play, see links to tools here

Thanks for the answer.

It is apparently a problem with the player (VCL).

I have now used the MPC-HC. That shows all covers.

you could add a file called folder.jpg to each alum folder and see if that makes the player display a picture.
There is an action to export covers so that you don't have to do it manually, one by one.