[EDIT--the more I think about this, the "solution" below doesn't make sense if the receiver displays "DD", unless DD can be 2-channel digital. My Pioneer receiver has all kinds of modes it displays, some being its version of sonic holography. I do remember solving that exact problem, though. Will have to explore receiver setup tomorrow. But it still might be worth a look at the Dish box setup.]
A lot of times TV dialog is mainly in the center channel. Commercials are often stereo. I have had that same problem, where it wants to only output to the mains. Seems like it happened when we switched from Charter to Uverse, had to set the Uverse box. Options: Audio/Language: Digital: Surround (i.e. not Stereo). But I don't recall the receiver saying it was in surround mode.
Had a similar problem when we bought a Visio TV (old Sony CRT finally shot craps). As shipped from the factory there was no sound at all, was ready to take it back until I started messing w/ settings. Had to set the TV up to think it had a sound bar, although it only had internal speakers.