I have put some songs onto my MP3 player but the songs are not playing. The songs are on there because they are coming up on the computer.
Either like others said the file format is not supported, or the mp3 is encoded in a way the mp3 player doesn't understand. Some mp3 players only support audio encoded in a certain manner and won't support certain methods of encoding like variable bit rate, high bit rates (320kbps+) , or mp3's with DRM. What it supports should be written in the manual, on the packaging, or hopefully on the product manufacturers website somewhere.