Providing the Operating System, display model numbers and the hardware used to interface to the displays as well as the manufacture type of any adapters would help troubleshoot this.
Most connection issues are due to people trying to connect multiple monitors using Display Port to DVI or HDMI adapters or Dongles. This video from AMD support provides a quick overview of the limitations of the technology of recent graphics engines and while specific to the Radeon line of consumer products is applicable to the FirePro series as well as they share the same chips.
This FAQ on the AMD graphics support site gives you all the gory details.
The Display Port to DVI dongle that is supplied with the card and many others available are passive (I.e. Do not contain electronics and do not require a additional USB port or a separate adaptor for power) and therefore only support single-link resolutions up to 1920x1200.
In general all displays should be the same native resolution and all should be connected using native Display Port connections. Other configurations are possible but likely require active dongles and or display port hubs.