Why, all of a sudden, will MacOS preview / finder not play mp4 files?

I've read other threads and none of them make sense to me. They're talking about codec issues and VLC being the answer; I couldn't disagree more. The mp4 files that are not playing for me were rendered on the same system. If my Mac can "encode" the mp4 it should be able to "decode" the file. This has been an issue for me for the last several months and I've not found any logical solutions.

I've been rendering and playing mp4 files on this same Mac. Now, when you try to play (spacebar) the mp4 file from Finder, it opens the 1st frame of the video and that's it; none of the play, search, FF or RW buttons work. What's the problem?