[Plugin: Viper's Video Quicktags] Most videos not showing up

I haven’t been able to make the plugin work, only very few select youtube videos are showing. I’m absolutely certain that I’ve used the correct urls so the problem isn’t there. Also on few occasions the video has first appeared, only to disappear a moment later and only displaying empty black space where the video should be.
I also tested embedding some flv videos from a website, and the same problem there: one video showed up, the other didn’t (from the same site).

If it’s of any use, this video was the only one that actually worked:

None of the ones on this channel worked: , nor did this one:
The theme I’m using is Station Pro and the WP version is 3.1