Lab Inputs Stopped Functioning Why?

This is weird. All of a sudden the left/right gain controls on the front of my NAD 2400 no longer adjust the gain/volume. The preamp signal does flow through to the speakers and the preamp can raise and lower the signal but I can’t adjust the gain/volume with the NADs controls?? It’s like they are being bypassed. Goofey enough I switched the preamp cables over to the Normal inputs from LAB and the NADs gain controls began functioning again, but when I switch back to Lab they stop functioning again?? Any clue? Thanks for your input.