Windows failing to recognize GPUs from second PSU

[thehun]

I currently have 4 Sapphire 7950s connected to my Z77A-GD65 MB, one plugged and 3 with powered risers. The plugged one and one of the rised GPUs are connected to the same PSU that powers the board, and the second PSU has pins 4 and 5 shorted and voltages are ok (measeured with multimeter). The fans spin but Windows just doesn't see the cards connected to the second PSU (the others work fine). Is there something I should consider?

I am using Corsair CX750 PSUs.

Cheers