Intel Motherboards : B150M-HDS problem

Pls. help. I bought a new B150M-HDS motherboard and it is not seing one memory module, i.e. it is showing that just one module (B1) is installed, although I have installed both (2*4GB). There is no problem with the RAM itself (either dimm works in slot B1 but doesn't work in slot A1). What could be the problem? Should I return the motherboard? I tried everything, please help.

Thank you?

P.S.: Could the fact that I am connecting a 4 pin ATX 12V connector instead of a 8 pin one have anything to do with the problem?