Interesting RAM/Mobo Problem

So:

I want to upgrade to 16 GB of RAM; not a problem right.

Before, I had two 4GB RAM sticks in bank B. Now, I have 4, 4GB RAM sticks, all the same exact model and everything else that matters.

Here's what's weird: All 4 sticks work but only in Bank 2.

Diagram:
<===================> A1 - Not working
<===================> A2 - Not working
<===================> B1 - Works
<===================> B2 - Works

Now I've tested all the sticks in bank 2 and they've worked in any combination...

Interesting RAM/Mobo Problem