Won't transfer music?

[skookum_gt]

I am a technological and iphone4 ignoramus so any help would be appreciated. I purchased some music from iTunes. It's in my library but won't sync with the phone. It says I need to authorize this device. I do that and it tells me that I am already authorized on this computer. I then try to sync again and it tells me I am not authorized on this computer again and the cycle continues. Have the newest update on the i4. Tried on girlfriend's iTunes and it works fine. Any ideas?

 

[Gregoris]

click on Store then authorise this computer.. it will then ask for your username and password and then allow you to sync all the tunes you want