4th 5830 not detecting - Biostar 890FX

[Genrobo]

I have two rigs, each with the same setup.
4 cards detect on one
Only 3 cards detect in the other
Naturally you'd think the failure point was the card itself, but I've individually tested all of the cards, including that one.
It's fine.

Could this be an issue with the motherboard? Or could it be the PSU?
Hard to imagine the PSU being able to pump enough juice for 3 cards, but not a 4th... Especially when I have the exact same PSU in my other setup.

PC Power and Cooling 950w Silencer MK II

[Lightspeed]

have you tested all the slots / is it a specific slot that isn't working?

[Genrobo]

Yes, one slot, but the thing is, that slot only doesn't work IF all 4 cards are plugged in, otherwise that single slot will work fine.

[J0k3r]

Do you have enough Power?