I recently decided to, somewhat belatedly, enter the 21st century by purchasing my first MP3 player. (�7.79 from Tesco!). I'd got some old BBC comedy shows, in MP3 format, that I'd purchased (on a CD) from eBay, which I wanted to put on it. The files played perfectly well on an MP3-compatible CD player, as they also did on my PC but, when I tried to transfer them to my new media player I experienced the same problem that you're having. (i.e. the files appeared to be getting transferred but they wouldn't show up on my MP3 player).
It eventually dawned on me that there's not a single recording standard for MP3 files. (There's something known as ID3v1 and also ID3v2. There might be other formats for all I know!). It occurred to me that the guy who'd made the recordings might have used a format not recognised by my MP3 player. So I imported one of the files into an audio processing program (Audacity) and I then exported the file as MP3 again (using ID3v2). When I transferred it to my MP3 player it was displayed and played normally. (I've since started reprocessing the other files, one by one).
So it seems likely that whoever recorded your MP3 files has used a non-compatible format. You can either seek freshly recorded files or spend some time converting your existing files to something which your device will recognise.
Chris



