I've been having issues playing some videos on the site, I click play and the screen goes black and then nothing happens. I've left it for ages in case it was buffering but never starts.
It's only on some videos and not all, but it's always the same videos that will or won't play and can't see any reason for it?
Using Firefox and have tried clearing the cache/cookies
If you have downloaded a video "offline" and it will only play sound but with a blank picture then it means your media player cannot play all the video codecs and file formats. The worst media player for causing this problem is Windows Media Player. Simple solution = don't use Windows Media and instead install one of the three better FREE media players, i.e. VLC Player, GOM Player or Pot Player. I use Pot Player myself because it has the best slide and scan controls.
If you are talking about playing a video "online" from a streaming site, such as UMD, then all those videos should play online perfectly, because UMD works like Youtube and they do not stream files in their original native format. All files uploaded to UMD are converted to MP4 x264 format and will stream the sound and video from your browser via their own MP4 player.
If the latter is your problem, then report the problem file names to Messmaster and he will test them for you, but they should all work because they are all in a uniform file format.