Are you using direct DisplayPort connections there, or are you using adapters, or a mix of both? Is it limited to a specific monitor or specific port on the card, or is it just the 6th monitor regardless of location? Have you tried swapping out the cable for a known-working one, if it's limited to the same connection/monitor? Have you ensured that you've got the drivers for the monitors, as well as installing any firmware for the adapters (if they're Accell/Bizlink)?
AMD Customer Care
The information presented in this document is for informational purposes only and may contain technical inaccuracies, omissions and typographical errors. Links to third party sites are for convenience only, and no endorsement is implied.