I have 2 x 750W PSU, each PSU runs 3 cards.
I dont blame Claymore 9.0 or 9.1, I had this problem since the first time I use claymore, since 5.0. There will be 1 card always not working, will hash 0 H/s. After a certain of time, watchdog says that GPU hangs and reset.
No matter how many GPU I use, there will be 1 not working.
My temporary fix was keep rebooting until all GPUs working, but now that ways seems not work anymore.
if you overclock your GPUs try downclock it little or increase core voltage. i use claymore zcash miner from the first version. never had problem but after v9.0 i have problem with psu and gpu hangs. after adding second PSU and changing overclock settings problems is gone