Need to find out about the setup then!
CPU?
USB 3 Risers?
Motherboard?
Number of GPUs?
Type of GPUs?
Ubuntu Version?
Do you have one GPU acting as the display output or are you using onboard graphics?
PCI-E settings?
6600K
MSI Z170 Pro
2x 1060, 2x 1050 Ti (USB3 Risers)
Ubuntu 17.04
onboard output (tried GPU but doesn't work either)
Gen1, 4G decoding
You need to have identical models for Ubuntu e.g. all GTX 1060 or all GTX 1050 TIs.
In your situation: you will be limited to using Windows 10.
Did you get all 4 cards installed or where some missing in Nividia Xorg Server?