I have the same issue, cant get more than 4 cards to work on the ASUS H270 Plus. The 4 1x PCI slots are working but soon as I try and hook into one of the 16x thats where it stops.  I can see the 5th card in device manager but has the orange triangle around it. 

I'm to the point where I'm looking at returning this MOBO and going with something else.  Shouldnt be this hard.