VBR is the problem. That was a quick fix! Thanks a lot.
I am using a Mac. First it was iTunes and that didn't work well. I downloaded Tagger, it got a few of the errors, but many are still missing the covers. I'll get my hands on a PC and try out mp3tag.
Many thanks CraftyClown.
No problem. If you have any in particular you are struggling with, feel free to link to, or email a track over and I can test it at my end and see if I can get it to work.