Noticed you were running an m.2 drive. Your issue could be that? While in theory you should have enough pcie lanes, in practice, it's possible your board is halving your gpu because of the other drives/hardware you have installed. I ran into this situation with my z270. It mattered which slot I installed my m.2: closest to the cpu and 16x on the gpu. When installed in the slot furthest from, because that slot shared lanes, I only got 8x. Took a minute to figure out what tf was going on. I suggest pulling the manual and seeing if any m.2 slots share lanes?