Is the second receiver also a HR21?. Even so, I am going to go out on a limb and assume the optical output path and destination are not identical to the one you are having problems with, or that the settings are not exactly the same on both.
Even though DD (actually AC-3, which is nearly identical) is the standard for origination of all digital broadcasts, it is poorly implemented, both in STBs and by broadcasters. TV stations usually go to upconverted SD (and its associated non-Dolby audio) during local content and commercial breaks, which can be problematic if they don't handle the metadata correctly, in some STBs. The tuner typically looks for a 5.1 flag, and if not found automatically downmixes to 2.0, but not everyone sets it up the same way, or even in a proper way.
This is one reason I prefer analog audio. It is also much easier to equalize and process for automatic level control, and I really don't ever hear a significant difference between most 5.1 and properly-done Dolby PLIIx 7.1, although many have convinced themselves differently, which is probably due more to wishful thinking than anything else.
If you have it work part of the time, you can rule out the cable or the connection or the routing. It that works at all part of the time, it is not the cause of dropout at other times, which is typically an improper metadata setting from the source or an improper setting on the AVR or display.
I would post this in a Denon thread on AVS. IIRC, there are some issues with interfacing DD with Denons, terrific as they may be.