3 of four cards recognized on asus P55A-UD3 motherboard

[Burner23]

Hi, I have n Asus P55A-UD3 motherboard which has 4 pcie slots. The board works with 3 Gpus via risers but when I add a fourth gpu directly, this new gpu is not detected.
If I unplug one riser, the fourth gpu is detected.

Is there any way to have 4 gpus on this mobo?
Will it work if i add a fourth riser? Or perhaps with a pcie splitter?

Hope you can help

[Burner23]

It seems that it was a weird combination of bios settings… After several attempts of rearranging the risers it suddenly broke the bios and after resetting it to defaults it recognized the fourth card… Strange. Issue closed