Local recordings on PC won't play back on Mac

[Bhrighde]

We have been recordings our meetings online and also on a local PCwhere we can choose the speakers/presenters (instead of all or nothing like the online saved videos).  We would then edit our videos on a Mac.

Over the last month or so, the files that were recorded on the local PC will no long play back on the Mac.  The files appear to be the correct format (MP4), but they can't be played or converted on the Mac.  They will play back fine on the PC.

I've tried converting the files to different video formats before saving them on the Mac, and they won't work.   I've also tried checking if there were any updates to the video players on the Mac (QuickTime and VLC Media Player), but we have the latest versions.

Would anyone have an idea of what might cause this? 

Thanks!