Okay, after looking at my set-up, here is what's working for me to get DD5.1 to my Yamaha receiver.
#1 - Seems obvious but it has to be said: Make sure the TV program and/or channel is broadcasting DD5.1. Not all programs on a DD5.1 channel are 5.1. For example, I am now watching a program on PBS local channel which I know broadcasts DD 5.1. The program itself is DD2.0 so the DD light and the ProLogic lights are both on with the L+R stereo light on. The other three lights are off. But change the channel to CBS, and bang... DD5.1 lights up, ProLogic turns off.
#2 - On the Dish 622/722 receiver press Menu > 6 > 7. Decoder= Dolby Digital / PCM. Mode= Line
My dish receiver is hooked up via optical connection directly to my Yamaha receiver through one input AND via HDMI to my Vizio TV with the optical audio out of the TV going to another input on my Yamaha receiver. Both inputs "see" DD 5.1 on the same channels meaning that the TV passes through the DD5.1 from the receiver.
NONE of the standard definition channels have Dolby Digital that I could find. I checked all the premium channels. HBO used to have DD5.1 on it's SD channel, but no more, I guess.
There also used to be a bug in the ALTERNATE AUDIO. When you had anything other than English chosen DD5.1 would not be sent from the receiver even if there was no alternate audio on that setting. This bug seems to have been fixed. I tried it on several channels. Again I first noticed that bug on HBO SD. not HD way back when DD was new to Dish.
Hope that helps