It's not bad power, that wouldn't cause one card to flake out. It could be a bad riser, cable or the little PCIE chip that plugs into the slot. I've had bad risers that run fine for days or weeks and then all of the sudden cards will drop off.  I hate to be obvious, but have you updated the BIOS and tried to set the Gen settings for the slots etc? I have one ASUS board and the rest are ASROCK H110 pro's. The H110's are setup for mining and  fired right up,  I have 6 10 card rx580 rigs running SMOS. The ASUS board is 8 cards and was kind of a PITA to get running. Had to set everything related to PCIE to Gen1, disable SATA controller, audio controller etc. Finally it has been running for over a week. I read ton's on boards before setting up my 10 card rigs and stayed away from the B250 as it seemed to have overheating issues and just be unstable compared to the H110 or another board. Hell, I paid $129 for my Asus Z270-P and it runs 8 cards...