moving windows across dual monitors

I have the display set to extend, have the monitors set to the correct order, and the mouse moves easily across the two monitors. But I can't drag a windows from monitor 1 to monitor 2. I did remember to reduce the size of the window, but while it moves over to the side of monitor 1, it does not appear on monitor 2. Suggestions for troubleshooting this?

What product and driver do you use?

Is it AMD's eyefinity, nVidia's surround, Matrox' powerdesk, etc?

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