How do I reset link between iPhone and iTunes on a Mac?
I have had similar problems with music and photos. Often, if you look at the files on the iPhone list via iTunes, they will be listed, but grayed out with a 'sync next time' icon next to them.

Somehow, these get in a halfway state, where the iPhone thinks they are queued for sync, but then they never do.

The only way I have found that resolves this issue is to first backup the phone on iTunes, then restore it, which wipes the phone, reinstalls iOS, then reloads your backup. When I do this, the sync list is cleared, and the files will sync as normal.

Good luck!