I just bought a new motherboard and I've been unable to boot with RAM memory into the dimm A-1 and A-2 slot. Everything works perfectly with the dimm B-1 and B-2, but when there's a RAM memory into the A-1 or A-2 the memok! light stays on. I did modify the timing and the voltage to match the RAM, and it doesn't change anything. All four memory sticks are the same brand / spec.
^ @tomemmerson yes they have to all be the same V as this is only a DD3 board.
I would suspect the motherboard might be defective if you're struggling with anything in DIMM bank A. I would try one module alone in either of those sockets, if you haven't already. (I would try 2 different modules, 1 at a time, in each of those sockets, one at a time)
Also, try clearing CMOS just so everything goes at default settings. Lastly, you can try to force the RAM to go slow (1333/1066), just as a troubleshooting measure.
If all that still results in the memOK light being on when modules are in DIMM bank A, then I'd RMA the board.