Why won't my MP3 player play a certain band's music?

so, I have a windows media player that says, MP3/WMA/REC/FM on it. This is similar to an ipod, but takes a AAA battery and has a USB port on it that is also a flash drive. All music that I have ripped onto my computer I simply transfer to this MP3 player like you would a flash drive and it plays music. When I try to load a 2 albums from WEEZER and it won't play their songs and the digital readout just says "unknown format". I have tried burning their music to a CD and then ripping it back to my computer and then transferring onto the MP3 player and still no such luck. The files that work and the files that don't work are both WMA.