ATI HD 7770 3 monitor set-up: running a 3rd monitor at a low resolution only

[CG147]

I also have a Sapphire HD7770 (ports: 2 dvi, 1 displaypot, 1 HDMI) - I have 3 monitors working - but the 3rd one will only run @ 480x640, I want it to run at 1200x1600.
Can anyone help?

I have the following set-up:
1: 30" HP @ 2560x1600 : DVI --> DVI
2: 20" HP @ 1200x1600: DVI --> DVI
3: 20" HP @ 480x640 : Displayport (+active DVI adapter)--> DVI

I have the latest catalyst drivers installed (windows 7 64bit).
I bought this card because I read in several places - it will definitely run 3 monitors. I guess I am running 3 monitors - but I don't have much use for a 480x640!
I can see that many people having similar problems hopefully someone has a solution - can anyone help please?

Update:
So I just discovered this. I can have the 30" plugged into 1 of the dvi's and one of the 20"s plugged into the display via the active dvi adapter - and both will run at full res.
Thought I had solved because - whenever I have plugged a monitor directly in DVI-DVI it has always been detected and then run at full resolution fine..... this time..... nothing. Not even detected.
This is getting really annoying - this card is suppose to be able to take upto 6 displays!!!
Anyone???? Please????

 

[CG147]

Solved:
Bit of strange one, but I swapped out one 20" monitors for a different one and now I have all 3 monitors working as they should:

1: 30" HP @ 2560x1600 : DVI --> DVI
2: 20" HP @ 1200x1600: DVI --> DVI
3: 20" HP @ 1200x1600: Displayport (+active DVI adapter)--> DVI

The "broken" 20" monitor when plugged into a single monitor system seemed to function normally. Odd, odd... very odd.