I figured it out. One of the rails on my PSU is malfunctioning. I moved the GPU power cable to a another one and it works now.
I have the same PSU and Graphics cards as you and am struggling to get all six working. How did you figure out that one of your rails was malfunctioning? Also, did windows 8 see all six right off the bat? Just installed 13.1 and they worked?
I'm using an A4 FM2 processor with a gigabyte F2A85x-UP4 but so far its been nothing but a pain in the ass lol. If you have 6 msi r7950 twin frozr working good in windows 8 with that msi board, I might be making a return to amazon soon lol