New miner here, two weeks in.  Running SMOS, we've been running 12 x rx580's for a week now at about 30 MH/s on all 12 cards with ZERO restarts.  I'm using a 2400W server PSU running on 240V.   When I try to plug in the 13th rx580 into the PCI-e x16 slot, I get a black screen and nothing else.  I thought I could just plug the PCI-e x1 riser into the end, but apparently not.  I've tried at least 6 different riser combos and nothing.  I tried plugging only ONE GPU into the PCI-e x16 slot and same back screen.  Any ideas would be GREATLY appreciated!