what's the model ? some mb are stubborn. I have an aorus elite v2 that at first barely accepted more than 1 gpu. I learned to know it , the x16 slots do not accept risers and I use them only with ribbon riser x16 and gpu directly connected on them. I've 2 m2 slots, the first works great with an adapter but the 2nd one never worked. I finally used a x4 splitter and now I've 6 gpus on this mb.
if you tried all settings in bios, maybe try to use ribbon riser, m2 adapter, connect gpu directly on your x16 slots and try the adapter x1pcie to x4 usb on one of your pcie slot that works.