Third ram stick won't work in neo 2 plat?

I have 2 512 ram sticks in my neo 2, a corsair 3200c2 and a mushkin, running fine.

I decided to grab another stick of corsair - same line, newer versions - and stuck it in my third slot (slots are CHA A, CHA B, CHA A, CHA B).

It won't boot. It gives the long wail upon posting, the one that tells you something is screwy with your ram.

I try the 4th slot, same deal.

I wonder if the ram is broken.. so I take out my original corsair and replace with the new one, again, 2 slots used. Posts fine. Okay, so the ram is good.

So I try taking my old corsair and putting it in the third and fourth slots. Same problem.

What's going on here? Do I have to do something special to use 3 ram slots at a time? Does it have something to do with dual channel?

Check your manual, but I recall it saying that the board will not run with 3 sticks of ram. I remember trying to run mine with 2x256 and 1x512 and it would not post.