It's possible you're using wma lossless, which can cause problems. You can try converting it to flac (flac is lossless so you won't lose any sound quality) by opening the terminal and typing:
avconv -i /filelocation/filename.wma -acodec flac /filelocation/filename.flac
If that doesn't work you could also try running a program called fre:ac on a windows computer (I used this approach) and converting it to flac that way.
Here's a forum thread of other methods people have used.viewtopic.php?f=47&t=25318