Either there is a glitch in the mechanism that prevents it from not opening or their is a problem with the motor or drive that moves the tray in and out.

Sometimes you can take the guts out of a regular dvd player/computer drive and drop in lieu of the one that came with unit and it will work. Trick is finding one that fits.

Otherwise , you might just have to get a new combo since repairing those things is not cost effective.