Okay, been through the manual - potentially things to check which might be causing the issue. From the Menu screen:
Under Manual Setup, Audio Setup, Dolby Digital Setup set to Off.
Under Manual Setup, Audio Setup, Auto Surround Mode set to On.
Under Input Setup, Input Mode, Decode Mode set to Auto.
Also, I think you need to make sure the amp is in Standard mode by pressing the Std button on the remote. This should mean when it detects the Dolby TrueHD signal, it automatically switches to that decoder. If it isn't working still after all this, while TDK is playing with the TrueHD soundtrack, try going to the Menu screen, then Information, then Audio Input Signal - this should indicate exactly what the receiver is getting. If it's standard Dolby Digital, then the issue must be the Blu-Ray player is not sending the TrueHD signal...