For long replays it can take a few minutes but 20 is far too long. If you take a look in the log, you'll probably find an error message there.
1.14.3 is no longer supported, iirc in some older versions the error popup didn't always open which would fit with what you're describing.
You might be able to find the raw (i.e. without start/stop/pause/resume feature) replay in ".minecraft/replay_recordings/raw/" and if it's there, you can move it up one folder and chances are that it can even be open in the replay viewer.