You've most likely got a problem photo, one that won't sync for whatever reason. That can be a corrupted file, or iTunes not being able to find the actual file, or who knows what. The Photo sync will give up on the first problem photo it encounters.
I recommend half stepping. Move half of the photos out of the folder, then try to sync again. If it still happens try syncing the other half. Once you've got the half that sync, add half the others back and try again. Keep adding and removing photos this way (halving the number that contain the bad photo) until you know which one it is. Even for a large number of photos this can usually be done in less than 10 steps.
It won't work well if there happens to be more than one problem photos.
And I'll be the first to admit there is probably a better way to do this, but my brain isn't cooperating on the subject at the moment.