RX480 and Rampage ii Extreme

I cannot get a 4GB or 8GB RX480 to work with my Rampage ii Extreme motherboard.  The screen just remains blank when the computer is started (but it goes through the boot sequence with a blank screen).

The RX480s work in other computers

The remaining computer bits work with other graphics cards.

I have flashed the BIOS to the latest and tried different card slots.

Any help or thoughts would be much appreciated.