There could be a number of problems. First, in your receiver, you may need to tell it that you have a center. If it already knows that, then you may have it's level adjusted too low. There is always the possibility that the center is not plugged into the correct spot on the back of the receiver (I've done it).
If you are only running the R&L (red & white) you cannot get discrete surround sound. You should be able to select a DSP mode, such as Dolby Pro Logic or PLII, that will give you simulated surround from a stereo signal.
If your DVD player and your receiver both support using a digital connection (optical or coaxial), this would be the recommended method for connection.
If you can answer Alan's questions, someone on here should be able to help you.