music not playing on iphone

I recently updated my itunes and my iphone to ios 5. Now, my iphone will not play some of my itunes music. It plays some songs and then just stops playing. The screen shows that the song is on Play, but the time counter doesn't move and no sound comes out. I tried stopping and starting the song, advancing to the next song on my playlist and even restarting my phone, with no avail. Not sure where the problem is coming from. All my music played with no problem before the updates. Ideas?