Problem running two sets of speakers

Sorry for the non-audiophile level question but I figure someone here knows the answer.

I just moved in to a summer house that has Niles in-wall speakers in the living room and kitchen, each room wired separately. The wiring to the kitchen runs through a Niles volume control unit (to control volume in the kitchen without having to go back into the living room). I hooked up a couple year old Sony receiver (nominally 100 watts per channel) wiring the living room as the A speakers and kitchen as B speakers.

Here is the problem. When I just run the living room (A speakers) alone, I get plenty of volume with the volume knob at about 1/8. When I run the kitchen (B speakers) alone, I need to crank it up to get volume even with the Niles volume control cranked up as well. When I try to run both, the A speakers die off to nothing while the B speakers are only minimally affected.

Questions: is the Niles volume control unit sucking all the power or is there something wrong with the receiver? If the former, is there anything I can do about it short of cutting the volume control out of the circuit (my wife likes the convenience)? Would better equipment help? Or am I doomed to only running them separately?

Thanks for any advice.

Bill