So I had the rig working with one MSI gtx 1080 connected to the x16 slot, when I connect a riser to the x16 slots (any of them) the card fans are working but the card won't be recognised by the operating system, replaced riser and still, doesn't work. The card is working of I put it in the x16 slot.

My mobo is an Asrock H81 pro btc r2.0 PSU is a Corsair Rx 750, in using a powered v3 riser, both molex on mobo are connected to same PSU as the riser and GPUs , on bios I have the pcie card as default, tried gen1 and gen2 everytime clearing CMOS after any plug, no way it is recognised by the pc. Anyone had same issue?