I faced this many times with setting up rigs. You could try installing one card at a time and check bios/windows each time, the moment you have a fail you should reset cmos and start from scratch.
The order in which you install the cards can matter greatly. Going in order from PCIE slot 1,2,3,4,5,6 often doesn't work. Try installing in different orders like for example 1,3,5,2,4,6