RoadHog : External Monitor don't work!

Hi! I'm Ryo.
On my Road Hog3, external monitor don't display...
I checked "External Monitor" on in ctrlPanel setup menu, but anything isn't displayed.
Ex. Monitor is certainly recognized, so palettes can be moved to ex. monitor.

 I have tried to :
Change BIOS battery.
Connect VGA cable oppositely. (both monitors certainly display)

So I think ASUS motherboard causes this problem...
(Ex. monitor is outputted from motherboard, main one is from video card)

How can I fix this problem?
Best Regards.