Go step by step:
First, connect only one card using the riser, and see what happens. (No other cards or risers.)
If i works fine, try the other riser cable, again, with a single card setup.
If either of these steps fail, try to plug the riser at different slots, and see if that makes any difference.
If that works too, then try one card plugged into the mother board, and one card with a riser. See what happens.
Then try only two cards into the motherboard, and no risers, and see again what happens.
Then try two cards into the motherboard, and one card with a riser.
Basically, you should try to isolate the cause of the problem.
So this is what i've done for testing, I isolate the test to only use the 1st PCIe x16 slot on the motherboard while trying on different different riser. From my observation, one of the cable seem to work better for the motherboard to detect the graphics card. The way I carried out the test was by having my boot-able HDD removed (so that I don't mess it up from constant powering on and off without proper shutdown), have a monitor connected to the graphics card and riser cable is used to connect PCIe x16 slot 1 on the motherboard to the graphics card (just 1 graphics card on the motherboard). I then power on the computer to see if it can display the boot-up screen on the monitor, with some of the riser the graphic card manage to display the bios boot up screen on the monitor while other just doesn't.
Here is the puzzling part, those risers that used to display the bios boot up screen on first try might stop showing the boot up screen after switching from other riser cable I test on or if I disconnect and reconnecting it (I'm certain everything is plugged in correctly between the motherboard and the graphics card and power supply cables are connected to the graphics card as well). There are times power cycling the motherboard could get it to show the boot up screen on the monitor while other times it just refuse to show anything on the monitor (the monitor is not showing black screen but rather it goes to standby mode thinking that the computer is turned off). Is it possible that the signal between the motherboard and graphics card is corrupted or the signal that the motherboard / graphics card is sending out is too weak for either components to pick up correctly?
To clear out the suspicion if the graphic card is not getting enough power from PSU, I'm using a CoolerMaster Silent Master 1500W so that is well enough for all 4 graphics card plugged in.