I had recently similar problem. My problem was that I had the rig with 7 GPU for quite some time and then I plugged 8th one. After ton's of experiments one of the major problems was that I did not switch drivers to compute on the 8th card.
Other common problems are:
missing GEN 2 settings in bios
pins not well aligned on M2 card - fix by reconciling M2 several times
Make sure you use latest BIOS - Asus recently released updated bios with some M2 improvements

good luck