I can't sync any music into my iPhone 3G

Question

In the case of Calendars, Contacts and Mail, they sync right. But when I try to sync my music, nothing happens. iTunes unmounts the iPhone in the exact moment that it is going to sync my songs.
The automatic sync I can't afford it, because I have 20 gb of songs in my Library. In the first sync, iPhone made immediatly a especial playlist in itunes, from which it is suppose to be sync from, right? I decided to arrange later that Playlist deleting some songs and let it below the 10 gb rate. And nothing happens, I have still empty the iPod app within iPhone 3G.
Any clues?
Thanx!

Answer 1

So I have found a common denominator to this phenomenon. After having the same problem, I started to recopy my playlist of 500+ songs in groups of 10-20, 30+, 75+, and then over a 100+. Any music I have purchased from iTunes won't copy, but everything else did. Unsure if I made a playlist called "Purchased" at some point or this is an iTunes default, I also noticed it had some of the songs contained in my other playlist and was showing on my iPhone. Regardless of what I did such as dragging music from that playlist to the other and clearing songs from that playlist, I removed it altogether from the iPhone itself vs. doing through iTunes. After that, music copied fine to my playlist.
So, my theory is that iTunes is getting tripped up on purchased tracks already showing on the phone and all songs are showing grayed out/dotted circle because iTunes can't resolve the conflict with purchased tracks.
Hope this makes sense and is helpful.