Most users would try the following, in this order of difficulty but above all try this when the Pixel is connected to a stable high speed WiFi connection.
- Delete cache of the MCH Android app. Video cache in a sync process is often corrupt.
- Try using h.264/AVC instead of h.265/HEVC codec in the Pixel.
- Update MCH Android app to the latest available.
- Update Pixel Android to the latest available.
If all these failed, try
- Downgrade MCH Android app to an earlier version that worked. There is a long thread discussion on this subject as to what version worked.
- Video not Synchronising to MyCloud home drive - #10 by tbone160
*> had a similar issue using my ibi with my pixel 7 pro ever since the server hack. Before the hack, my videos (no matter the size) would backup. Now I can’t even get a 10 MB video to backup. Note, all photos do backup.