Some Videos Not Copying to iPad

I imagine you have re-booted.

• Have you tried a forced restart of the iPad?

Force Re-start iPad

This is different from a simple Power Off. I had to continue holding the button down past the "slide to power off" message, and then I released the button only after  the Apple Logo appeared indicating a re-start. 

• Re-start the Mac in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Use another library if you have one, or make a small test Library with videos in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Make a new user and see if the same problem recurs with it
• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon.

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…

It is true that transfers can be interrupted when an incompatible file is encountered-- and often with no indication of why. Videos are major culprits. Being "all the same format" doesn't really mean they're all the same format: ".mov" in one movie isn't necessarily the same as ".mov" in another, for instance. Videos are like from the Wild West.

I'd try transferring the videos one at a time to see where the culprit lies. Let us know what happens…