Issues with Mobo ASUS z170 pro gaming : can't detect 6 GPUs

I decided to build a new rig, and was struck by a compulsive buying disorder :
instead of going for a standard asrock BTC PRO, i bought a skylake Asus Z170 Pro Gaming Mobo along with a pentium G4400, 6 GPUs and a PSU big enough to power everything.
My problem is the following : no matter what Os i use (tried Win10 / xubuntu 15.10 / xubuntu 14.04), my mobo is unable to detect all my GPUs.
At best i got 3 GPus working on each OS.
On windows 10, the mobo even recognized up to 5 GPU at one point, but 4 of them weren't working (This error showed up in the device and hardware monitor : This device cannot find enough free resources that it can use. If you want to use this device, you will need to disable one of the other devices on this system. (Code 12))

I've also tried to change the Mobo's BIOS, without any noticeable changes. I checked my risers one by one, no problem on this side.
I also noticed some erratic behaviors with the mobo : for instance, sometimes the keyboard / bootable USB stick wouldn't be recognized during the boot, with all GPUs connected to the mobo.

The question is : should i send the mobo to RMA or am I missing something here ?

Thanks,

Cubirez