Ok, what do you mean when you say they don't work. Do they not fit? Not power on? etc.
Also I don't think they make native 8 pin connectors for GPUs, well they might but it would be a really dumb thing to do seeing as how 6+2 would be more universal.
As long as your not mixing up a 6+2 connector with something like a native 8pin cpu connector or a 4+4 pin cpu connector it should work just fine as it is.
i believe there is not enough power, 6 pin part work fine but when ever the 7970 is plugged in using the extra 2 pin part the MOB beeps 1 long 3 short and VGA LED stays on i wanted to try a complete new cable to the GPU