Songs I Purchased Won't Sync To Phone

Okay so here is the story. I was running and iPhone 3G and I had purchased quite a few songs from iTunes. I transfered my purchases to my netbook and they all play just fine. Well I just got a 3Gs (hand me down from my mother who got the iPhone 4) now today I looked through my music and realized one song wasn't synced to my phone although another song by this band synced. All the other songs I purchased are syncing to the phone just fine. I've had this issue with a song before and I ended up having to delete it and reload it from the CD.

Does anyone know WHY this happnes and what I can do to possibly fix it without having to buy this freaking song again? I'm a college student and I can't afford to keep re-paying for songs I alredy purchased. Any help is greatly appreciated.