the probable reasons are 2. Most dvd trays are driven by a belt. To make sure that the belt is not slack put your ear on top of the dvd player and see if you hear the motor running freely when the tray is fully closed. If so then open the thray and pull the mains plug before it closes back. Remove the cover of the player, push gently the tray and through the hole in the center of the tray you will see the belt and the pulley. If the belt looks loose replace it. When you do buy the belt take the old one as a sample and get a slightly smaller one. Not too small otherwise the motor will be strangled and will not run.
Another reason is that all dvd mechanisms have 2 trip end swithes or optical sensors under the tray.
You will have to remove the mechanism from the player. Be careful not to damage flat cables and make sure that the tray is not fully closed by removing mains plug before it closes back. Pull the tray fully back once it stops you will see two plastic clips that prevent the tray from getting out. Using a small screwdriver try to force gently the clips while pulling the tray. Once you have done this you will see push switches. You will need a contact cleaner spray. Spray the cleaner on hole of the switch push rodes wait two three minutes then clean up everything and assemble. Be careful not to thouch any mechanical part when you clean.
GOOD LUCK.
Simon Efrem
July 2010