2 x AMD W600 graphics card can only power up to 9 displays on my video wall ?

I have installed 2 x AMD W600 cards on my server to power a Samsung Video Wall of 12 displays of 50 inches. It seems like the 2xAMD W600 graphics card, can only power up to 9 display. I am using Mini-DP to HDMI adapters. The other 3 displays cannot detect any HDMI signals from the card. Anyone has any recommendation ?