what board do you have? also are your powered risers sharing the same molex lines (as in separate lines to PSU)? i had one rig that ran fine for days then i messed with it and it wouldnt boot. all cards boot fine but when combined no boot. put the risers on separate molex leads and it worked fine since. seasonic 1050w power supply. whats crazy is i have some system using ocz 850w psu's and have them doubled up on molexes with no problem at all.

also if your psu is not single rail it could be pulling too much power from one rail. i would suggest a psu issue if its not posting. also i bought some risers cheaply made and the yellow would push partially or fully out when you plugged it in so i just plugged that lead directly into the psu molex for a tight fit