It won't sync properly anymore. I constantly sync movies and tv shows to my phone to watch at work. Never had a problem before. Last time I did it was two days ago. Had a day off from work, went to go sync it with more videos but it doesn't copy them over.
iTunes goes through the usual motions. "Preparing to sync, determining apps to sync, etc". It never mentions copying over more files, and the files already on my phone won't go away unless i delete them manually from my phone. If I untick them on iTunes, the files are still there. The sync completes without any errors, yet there isn't any change to what's on my phone.
My first guess would be a software issue with iTunes.
That's why my first advice would be to check for updates for iTunes. If iTunes is uptodate:
Completely remove iTunes and its components from your computer, using http://support.apple.com/kb/ht1925 for windows XP or http://support.apple.com/kb/HT1923 for windows Vista or windows 7, and afterwards reinstalling iTunes and try again.
If you use Mac, simply download and install a fresh copy of iTunes, no need to remove anything.
Keep me posted about the outcome