Have you checked the setup menu on the DVD player. As cb1 said earlier, the Digital Output is a feature that often needs to be turned on in the DVD player's setup. This only needs to be done once.

If you've done this, then check on the actual DVD disc's menu and see if it has different audio settings.

If these two prove to be set OK, check to see that you have selected the digital input mode on your receiver. Sometimes their is an input mode select button, sometimes you have to assign the digital input to a particular source (in your case DVD) in the setup menu of the receiver.

If you can't work your way through the above, post the actual model numbers of your equipment, and we might be able to track down some specific setup information.