I didn't notice this at first but I've just realised the player isn't even listing quite a lot of the folders containing MP3s on my USB stick when I press search and scroll through them. I know there are bitrate limits on individual MP3 files but I haven't read anything about any restrictions on the folders themselves. It explains why the shuffle was never playing any songs from a third of my albums anyway!
Any ideas what could be happening and how to resolve it?
thanks