I figured out what was causing the problem.
My receiver has a setting for "Surround back decoding mode [SB DEC]". I had it set for SB Auto, figuring it should know how to decode the sound to the back speakers better than me. I was wrong. The correct setting was/is SB OFF.
The manual is confusing on this setting. How shocking!
"By decoding the surround back signal of DVD software (etc.) recorded in Dolby Digital Surround EX, DTS-ES, Matrix, DTS-ES, Discrete 6.1, etc., format, you can enjoy the surround sound intended by the filmmakers. Select the surround back decoding mode using "SB DEC" on the SUR menu.
SB OFF - Surround back decoding is not performed.
SB AUTO - When the input stream contains the 6.1 channel decode flag, the appropriate decoding is performed on the surround back signal."
Thanks to any and all of you that read my posting and tried thinking of how to help me.