Intel Motherboards : Z170 Gaming ITX - Memory Issues

Can't get system to boot with the second DIMM slot with the GSKILL TridentZ 3770mhz DDR4 that I've got. Have checked each stick in Slot 1 and boots fine, single stick in Slot 2 and it won't boot and neither will sticks in both Slot 1 & 2.

So far I've tried:

1) Downgrading to 1.60 bios

2) Upgrading back to 1.80 bios

3) Reseating CPU (and checking for bent pins)

Any thoughts?

Scott