Had the same issue with the B250, use the 24 pin connectors only, skip the molex it does something weird to the PSUs, I had to return on PSU cause I couldn't get it operational.

My working setup with b250

• 3 x 1000w PSU - All 3 atx 24 pin plugs in the board (this daisy chains the mobo/psu on off cycles)
  PSU A - powering mobo 24 and additional CPU port on mobo as well as the ssd hdd, 4 GPUs and risers
  PSU B - powering 6 GPUs and risers
  PSU C - Powering 3 GPUs and risers, I have 3 slots/cards waiting on ASUS to unlock the BIOS to enable beyond 13 GPUs...
  No Molex connections