Since downloading, after inserting movie DVDs, they are ejected shortly after. How can I remedy this?

Yesterday I decided to watch some DVDs all was going great and I decided that now i had money i would finally download Mountain Lion OSX. The download went smoothly, however when I decided to start watching some more films, after about a minute of the film playing, the DVD stopped and was ejected from the laptop, without me doing anything to it. Please can someone help, as I am a university film student and this is the only DVD player that I have! Thanks! Alice