It's been a coupld months, did you solve your problem? If not, a few details on your setup may help. I assume you have never achieved 6 monitors running, only 5 working so far.
Connections: Are you using passive cables on any of the miniDP output ports? Be specific: how many have passive cables (and if so, are they miniDP-to-DP?), and how many have active adapters (and if so, are they miniDP-to-HDMI or something else?).
Monitors: Are all the same? Are you using DP, miniDP, HDMI or other inputs? Uniform for all or differing on some? Any DP-MST daisy chaining?
Specifics may help for others (like me) to compare setups and offer advice.