Miners dogpiling on one device
You could check the temperature on the devices, the one active should be running much hotter than the others.

If they're all running hot the bottleneck is elsewhere (CPU or memory/bandwidth). If none of them are running hot you've bought one hell of a CPU to get 200Mhash.

Else it's advisable to check whether or not you can use the non-default videocard at all.

I'm afraid I lack the videocards to test this myself :)