Please check in Advanced--> Onboard Device Configuration--> PCIEX16_ slot Bandwidth.. and see if x4 mode, x2 mode and auto are in options.
if its present.. put that in x2 mode and see if 5th gpu works..
I don't see that option. I see these options:


I only see PCIEX16 info on Tool Tab. Here are some images.



The raisers are plugged in:
PCIEX16_1 = Asus Turbo GTX 1080 - 8Gb
PCIEX1_1 = Gigabyte Turbo GTX 1080 - 8Gb
PCIEX1_2 = Gigabyte Turbo GTX 1080 - 8Gb
PCIEX16_3 = MSI GTX 1050Ti - 4Gb

check this youtube video.. might help