RAM Detection/Display Signal

So, I have a Asus Strix x370f and I recently discovered that from the 16GB RAM (2x 8GB 3600 16 TridentZ RGB) only 8GB are available. They are installed on A2 and B2.

I tested the first Stick in the B2 slot, 8GB fully detected. Second stick, B2 slot, 8GB fully detected.

However, when I tried this only with the A2 slot, I have no display signal. My MOBO manual says that you need the A2 B2 slots for dual channeling. Using B1 B2 the full 16GB are getting detected and available. Using A2 B2 16GB detected, 8GB available. So the problem is at the A2 slot.

The RAM-Sticks are 100% placed properly. MOBO has been flashed already, latest drivers installed.

I have no ideea what to do. Any suggestions would be greatly appreciated.

EDIT: Resource Monitor says: 8259MB Hardware Reserved??