iTunes won't fully sync with iPhone, iPod

That's the thing that's being going on for while but I haven't had the time to look it up. Basically I've noticed that my iTunes library has lost some tracks (I can't remember which for sure, but I know I had more music than I currently have) and that seems to have happened because I did transfer my library several times (from one PC to a Mac, then from a Mac to another Mac and then to a third Mac). On top of that, some playlists do sync fully either with on my iPhone or iPod. For instance: I have a playlist here that has 11 tracks on my Mac, but in my iPhone it has only 8. I've resynced a few times and still, it doesn't sync. And its the same with several playlists, so recreate them would not be practical. I haven't rebuild my iTunes library (trying to figure out how to) but is there anything else anyone would suggest?