If an MP4 won't play on a phone despite working on a PC, it may be using an unsupported codec profile or missing a "moov atom"—a piece of metadata that needs to be at the start of the file for streaming.

Technical users often use FFmpeg to mux or demux files with precise control over codecs and metadata. 🛠️ Troubleshooting Common MP4 Issues Supported media formats - Android Developers

Incomplete downloads of live YouTube video recordings. #14780

I recorded a meeting but forgot to press "stop recording". now i got a mp4 file with 0 bytes and a .webex file with with ~300 MB ( Cisco Community