ASRock z97 extreme9 cannot fit more than one GTX1080ti

[sciencefans]

I updated the latest BIOS version, inserted 2/3/4 msi GTX1080ti, and entered the system (win10) with latest NVIDIA driver(384.xx), then after a few seconds, the screen froze and the fans of all GPU cards squealed. Finally I got a BSOD.
However if I insert only one GTX1080ti, everything goes fun. And if I insert 4 TITAN black, nothing wrong happens.
Could anyone give me some advice?

 

[sciencefans]

Okey finally I get a middle way. Two 1080ti works well in slot 2 and slot 3 on that motherboard. Neither 1 and 4 can be used by 1080ti. Quite strange.