Asus P5Q won't POST with RAM is some slots

Hey, all -- got myself a P5Q PRO with a Q6600 processor and a single 2GB DIMM of Muskin DDR2-8500 (1066 MHz) RAM.

I put the DIMM in the B1 (yellow) DIMM slot, per Asus's recommendation. All was well. I ran XP 32-bit just fine for weeks.

I decided to upgrade to 4GB of RAM and switch to Vista (also 32-bit). I bought an identical Mushkin 2-GB DIMM and put it in the A1 DIMM slot, figuring I'd also get dual-channel out of it.

It won't POST. I tried various configs, and the only one that works is to have both DIMMs in the B slots (B1 and B2, yellow and black). If there's a DIMM in one of the A slots, the computer won't POST.

Vista sees 4GB (well, 3.5), but it's not dual channel.

No overclocking here -- I reset my BIOS to default values.

Any ideas what's going on? Is it a bad mobo? Am I missing a jumper that I need to set? Help! (And thanks!)