If it was me who was having such issues, I would use graphstudio
(free app) to render the file in question and see what codecs/filters/splitters were being used now. This would likely give some clues as to why the file is not playing.
Then I would use Sharks
settings app to adjust and try different codecs/filters/splitters for that particular file type, till I found something that worked with the player I wanted to use.
To render a video file in GraphStudio (GS), simply open the app and drag and drop a video file of the same type you're concerned with from explorer onto GraphStudio. Then you will get a visual graph of the specific codecs etc that are being used by your system to render (play) the file in question. After you make a change with Sharks settings app, you can render the file in GS again to see if the codecs being used have actually changed.
(I also would not be using WMP, but that's just me. I like MPC-HC better.