How come only some of my songs play?

Hello, i just bought My Sansa Shaker 1 GIG, on sunday and when i uploaded my songs, only some of them would play! i took everything off, and reloaded them, but still the same thing, can anyone help me?

the shaker only supports MP3 files. make sure some of your songs are not WMA or other file formats.

I just bought a 512 MB Shaker and I have the same problem, all are MP3 format only and in fact I have removed, played on PC and then loaded again, the same set of songs wouldn’t play, any help??