I just got an H81 Pro BTC and installed windows using the igpu then I tried to install my RX 480s and they didn't work.

So far I have tried:
plugged in both the molex connectors
Clearing the cmos
reFlashing the bios
Tried to put the one 480 directly on the 16x pcie connector
Set pcie to Gen 1 and Gen 2
Tried different risers
Tried different HDMI cable

I did actually get it to work and got really excited, I then installed the drivers and restarted, but then it stopped working.

Do i have a defective mother board?

My rig:
1600w EVGA PSU
6 RX 480
Intel G1840
4gb ddr3 ram
H81 Pro BTC
6 powered risers
128 gb ssd

Any help would be greatly appreciated!
Cheers