mplayer problem

[toyfactory]

I don't know whether this is the right place for this question but...

Mplayer seems to work ok for playing DVDs and .avi video files from the
hard disk, but fails to play the same video files from a mounted DVD. 
It just sits there with the drive grinding away as it it's trying to
read a damaged disc (the disc is fine, it mounts fine and I can access
the files on it no problem).  I had a quick search through the forum but
couldn't find anything.  Any ideas?

Nick