I had all sorts of issues on Win10, switched over to fullzero's nvOC and its working like a charm. super easy to use and supports the nvidia 10 series. If you have an extra 16gb thumbdrive you can image it over and boot it up off the thumbdrive and see if it works without formatting your Win10 off whatever you have installed on

Are you using a prewritten .bat file, if so check the cuda_devices and make sure it has 0 1 2 3 4 5

Also, this might be a stupid question as i doubt it'll let you boot without it after 3 cards but worth checking all possible problems - Did you plug in all the molex power cables to the board terminals?

in the past i had to ensure 4g encoding was enabled and then play with the PCIe settings (trial and error with Gen1 settings, Gen2 settings, etc).