Why some videos (all MPEG-4) cannot be synced to iPod classic while others can?

Why some videos (all MPEG-4) cannot be synced to iPod classic while others can?

Sorry, I just posted above question which is too brief. I would like to revise it as follows:

I have used my iPod Clossic for years and never had any problem - until I downloaded the new iTune (11.0.0163) yesterday.

I synced my iPod Classic to the new iTune exactly the same way as before, but only 23 of the total 171 videos were synced to iPod, in a random manner. All these videos were either originally MPEG-4 or converted to MPEG-4 before input to iTune and synced to iPod Classic before with no problem. Can someone help how to revolve this problem?

I also have a iPad2 and a iPad mini which already have all the items synced, Now with the problem on iPod Classic, I dare not even try to sync my iPad2 and iPad mini to the new iTune, fearing it may wipe off all the videos I have already synced in them.