Denon AVR-1800 receiver issue

I have a Denon AVR-1800 home theatre receiver. Recently, when I play a DVD, it has failed to recognize dolby digital DVDs as digital. Instead, in the "auto" mode, it only plays the DVDs as if it is analog (or Pro Logic). I cannot figure out how to bypass this mode or make it recognize my digital DVDs as such so they can play through my surround sound speaker system. Any ideas out there?

A one question wonder I guess.
ANYWAY check out the output of your DVD player, its probably PCM instead of DD:1:

Check for an analog/digital button on your remote.

Thanks for the responses. I should have added that my Denon system has recognized my digital DVDs from my Sony DVD player for many years without issue, but it has only recently played the digital DVDs as if they were analog.