iPad Pro won't sync or play my mp4 files

I have a number of mp4 videos which used to play on my iPad, and when i replaced it, they played on my iPad Pro.

I cleared my iPad Pro last week and reinstalled everything from scratch, and now it won't play the mp4 files.

I'm using Photos, Version 6.0 (361.0.100),

File types for example MPEG-4 movie, H.264, 640 × 360

iPad OS14.7.1 (18G82)

The error message says 'a file' (and it doesn't tell me which file, but i have narrowed it down by deleting files!) 'will not sync because it cannot be played on this iPad'.

It doesn't give the file name but instead a string file name that my mac will not find - for example “FA75BB17-83B6-4901-9752-A300E41AEF90.mp4” was not copied to the iPad “iPad” because it cannot be played on this iPad.

I've tried copying the file to the desktop, and running it through Total Video Converter Lite, to make it H264, or iPadPro compatible, but just got the same message when trying to sync them from Photos to my iPad.

I really don't want a workaround for this - like 'drag them straight to your iPad', because there are many videos and they are all part of presentations, which are kept as folders in Photos, to be played on the iPad as a series. So its a file type fix that i really need, not a workaround!

Thanks in advance

Paul