also, I believe if the ATV is already using a significant amount of network bandwidth (like streaming/downloading a movie), syncing will wait for the task to be completed.
Another one then ... does anyone else have problems with artwork? iTunes tells me a video file has one artwork and on the AppleTV is shows another (a previous artwork for that file). Others that were changed at the same time are showing correctly after a sync, but some don't.
I also have 1 file that loses it's artwork each time I edit it's description .. annoying, the only .mov file in my library if that has any bearing on the matter.
To fix your out-of-sync movie art, remove the file from iTunes, add it back, then hit the sync button. The "old" art was cached, and this process deletes the old art from the cache.