Videos synced but cannot play on phone (some can some cannot)

I downloaded videos, MP4, and synced them onto Iphone 6+ with itunes. 4/6 of the videos can be played, while the other two cannot.

Usually, when I click a video, it enlarges and starts playing.

But for those videos that can't play, after I click them, they enlarged, then flicks to this page:

The little loading circle on the upper right corner just kept on rotating. I thought that means the video is loading, so I waited for 3 hours, and it's still rotating, nothing changed (I kept the phone open, not locked)

I tried resetting but that doesn't work.

Does anyone has the same problems as me?