Yeah the alternate I have seen when people run into weird bugs like this (and cgminer may not fix it) is a re-install of os & drivers.
You may "work" on it for days without accomplishing anything. The more direct answer is there is no hardware limitation that would cause that. It is screwed up OS/drivers/OpenCL SDK. My guess it is a windows box and when they get "hosed" they tend not to fix themselves. He can try uninstalling the drivers and reinstalling, and trying to copy dll and remove cards, and do reboot, and use driver cleaning software and a couple weeks later give up and reinstall OS and it will work perfectly right away.
I recall someone else having similar issue recently and indicated problem went away after a re-install.