If it worked with 5 before and windows is detecting 5 still in Device Manager you probably just need to uninstall drivers with DDU (in Safe Mode) and then reinstall your drivers. Hopefully that will have all 5 working again.
I didn't have 5 cards before. I had 4 cards running previously, now that I have 5 I was hoping to get them all to work. Even though window's sees 5, one has a with triangle on it. The triangled card is was also working previously.
Driver's were reinstalled with DDU in safe more many times.
I also just installed Bios version 3 too. I'd have to say that installing all of your cards to the mobo, running DDU, then doing a bios update was the easiest way i had to get these cards recognised without doing some monkey work.
Any other suggestions for this last GPU?