Weird. What kind of setup do you got going on with risers? I wonder if they are getting enough power. When you boot up, does it spin all fans on all the GPUs?
I just tried with a GPU on the 16x PCI-E + 4 GPU on riser. It's now working....
WTF is this.
I have 4 cards on risers right now, with no issue and another 2 are being put in tonight, so something is not setup right, I'm just not sure anymore without being there in person.