Has anyone encountered the following problem?

I am unable to use PCI-e 16x to 16x (powered) riser cables on a motherboard. The MB in question is Asus Striker 2 Extreme (with latest BIOS). I tried all three PCIe 16x slots with two working riser cables and three different graphic cards, all that happens is card fans spin up to max. There is no picture (if used in Slot 1) or the card won't be recognized in Windows. The PCIe slots do work without riser cables.

What difference there is between using a riser cable and not using one, from MB's perspective?

Is there anything I can try to fix this?

Is it really necessary to use 16x cables on 16x PCI-e slots?