do you using powered risers ? the risers connected to the motherboard on the right way ? are you powering the risers and the cards ?
The 2 molex connector on the motherboard are powered ? connected to the psu ?
when u using one gpu (rx580) , you connecting that gpu to the PCI-E x16 3.0 Slot via riser ?
Do you connected the gpu via riser the first time when you tried the setup the gpu ? If yes are you sure you plugged the riser on the right way to the motherboard ? if you plugged the riser on the opposite way , you might killed your gpu ...
The right way to connect the riser to the mobo :