Question about my phone....?

I have a Samsung Blackjack, and it hasn't been working right. I download music from Napster to my phone, but it won't play. It will only play the saved music that I transfered from my computer to my phone. Whenever I try to play a song that I downloaded from Napster, it tells me that there is a digital rights management error. I've tried turning off my phone and taking out the battery, but nothing is working. Does anyone have any ideas what could be wrong with my phone?