What options (config file, parameters...) are you using? Any undervolting/overclocking/etc.. ?

Also, how much VRAM does your cards have? At this nfactor, VRAM could be a limitation, sometimes solvable by increasing the lookup gap.
As a blind shot, try increasing the lookup gap and see if that squeezes more hashes out of it.

If that does not work - please post your configs/params you use, you may get a better advice once we see them