Ubuntu 16.04 mine with only 4 cards

[Cry2Mine]

First hello to everyone.

I try to build ethereum mining rig with:

6 x Asus Dual GeForce GTX 1070,
BIOSTAR BT250-BTC
PSU be quiet 100W
CPU G4560
RAM 2x 4GB

What I do?
I connect all parts together and set the motherboard in "Mining Mode". When Ubuntu 16.04 installation is completed I switch to NVIDIA driver 375.66 and reboot the machine.
After the reboot I can use only 4 from my 6 GPUs.
When I type

lspci -v | grep "VGA controller"

I see all 5 GPUs but on

ethminer --list-devices

or in NVIDIA X Server settings  I see 4 GPUs.
Can someone help me with a hint what should I do.