Advice on mobo 970a-ds3p

Hi guys im building a new rig, i have 3 with this mobo from gigabyte, running just fine, but this one im having problems, only 3 of the 5 cards get detected at the same time, i tested every gpu and every riser, one by one, and everything works fine, the problem is when i connect the 4th and/or 5th gpu, any advice?

I read somewhere about shorting the pci express slot? thanks