I believe there's a bug with this plugin. If there's something that interrupts the event processing, like a wait command or text box, videos won't play even if they're loaded.
Here is how to recreate the bug
1: Make a new game project.
2: Only use this plugin.
3: Make a map event, can be triggered by anything, but for this example make it triggered with the action button.
4: Use the plugin commands or scripts to preload a video, set an id, then play the video.
5. Start a new game and trigger the event: the video should play normally.
6. Go back into the project and put a "Wait" command of any length in between the preload video event and play video event; for this example set it to 60 frames.
7. Start a new game and trigger the event: the video will not play.
This is using the most up to date version of the plugin 1.2.0.
Edit: I tested with both mp4s and webms. The problem persists even between the two.
Edit2: I tested using the old 1.1.0 version and it did not have this problem.