The rig boots fine, whatever the number of GPUs.
It's just that one of the GPUs is not recognized by the driver. I can still see it in lspci, though.
CPU is Intel(R) Celeron(R) CPU G1840 @ 2.80GHz
Update: I replaced all GPUs with 750Tis. 6 of them. And they all worked fine.
Another pointer towards the PSU: a Corsair CS750
I have similar issue before except that mine is sixth gpu did not work. Finally I made it by switch gpu with different slots and ddu and reinstall the drivers. What is the problem in your device manager? there should be some error code. 43 or 12.