cpu2 dimm all disabled

Hi

server = R710

I have 12 x 4gb total ram. 6 x 4gb each CPU dimm.

when booting up bios shows CPU2 B1-B9 slot all disabled so in BIOS only shows 24GB memory.

Could it be motherboard issue? i have confirmed it is not due to faulty memory. 

alternatively can i fill 9 x 4GB to CPU1 dimm and leave CPU2 dimm blank? In windows i still seeing two processors though.