When the card is not accessible do you get a yellow triangle sign in your device manager (if you use windows 10 e.g.)?
I hade some gtx 1070, they worked from time to time - then again they didn't.
After some reboot they worked then they didnt etc..
Replaced the card, everything fine.. :-(
I do use Windows 10. The card is powered up (I can see the lights on the card) but I dont see it at all on the device manager.
If I disconnect one of the other cards, I can see it so I know the card is working but Im having trouble getting all three to work at the same time.