Windows 10 not booting with 2 sticks of ram

[Xcept1]

Hi chaps, slight problem here. Building a new system for a work friend but Win10 doesn't want to boot up when I have 2 sticks of ram in at once. On their own they both work absolutely fine and it boots up without any problems but as soon as I put the other in....no joy.

Mobo: Asus Maximus VII Ranger
Ram: 2x 8gb 1866MHz DDR3 HyperX Fury Red Series

What am I missing here?

 

[kayomani]

I suspect you might either be using the wrong slots of dual channel or somehow the system bus is struggling on the voltage as its essentially overclocked ram.

 

[Xcept1]

I've only tried using the 2nd and 4th one so far.