I took a few quick shots to show you exactly what I am seeing on my system.
Most are self explanatory given their file names. As you can see, the 921 has a lock on my local digital ABC but receives nothing but snow when I manually enter the ATV channel (27) into the 921 (an auto scan returns no channels at all). To further illustrate this, I have taken the lead from the 921's off-air antenna input and have connected it directly to the coax input of my Mitsubishi HD-ready set.
One of the few DTV channels that I can get a lock on
The corresponding ATV channel as seen through the 921's off-air tuner
I am able to receive nothing but snow on any of the corresponding ATV channels when using the 921. When the same antenna lead is connected to my television I receive strong signals w/great picture on all ATV channels except channels 15 and 47 which are less than perfect.
Here is that same station with the off-air antenna connected directly to my television.
As of this morning, the 921 is only able to receive a solid digital signal on two of my local DTV stations (ABC's WKOW-DT and PBS's WHA-DT). I've included a photo of my unsuccessful attempt to gain a solid DTV signal on DTV ch 50 (CBS's WISC-DT channel 3). The signal meter jumps from 0 to 70 but does not lock. I have this same signal lock issue with DTV ch 11 (FOX's WMSN channel 47) and DTV ch 19 (NBC's WMTV Ch 15).
inconsistent DTV signal unable to lock on
...and a once again a clear picture of the same channel with the antenna connected directly from the diplexer to the antenna input of the television.
In addition to this, my 811 connected to an HDTV in another part of the house receives all ATV and DTV signals (with the exception of NBC which is intermittent).
I don't know if this helps, but it may prove something to you. If I can receive such a clear picture from the diplexer-TV connection, shouldn't I be able to receive a similar signal on the 921?