Forgive me for asking the obvious questions first.
Are you certain the disc said "Dolby Digital 5.1 surround," as opposed to simply saying "Dolby Digital"? Some discs are Dolby digital 2.0 only.
Many surround discs don't have that much information directed to the surround speakers. Did you put your ear up close to them to see if you could hear any ambient sound coming from them, or merely dead silence.
Did you go to the DVD's menu, and select "setup" or "languages," or something similar, and select the Dolby Digital 5.1 surround option? Some discs default automatically to this selection, some don't.
If you have been getting good strong Dolby Digital 5.1 surround from other discs, you can just disregard the following dumb questions. Without knowing your level of expertise, or your past experiences, it's a little difficult to know what to suggest investigating.
Is the receiver's digital input into which your DVD player is connected (Optical #1, Coaxial #2, or whatever) assigned as the receiver's DVD digital input?
In your receiver's speaker setup menu have you told the receiver that you have surround speakers?
