weird focus problem with pyqt

I have a gui built using QT and Python using pyqt. I use several widgets linked to methods using the usual signal and slots mechanism. For some widgets like certain push buttons or radio buttons (not all of them ..) after I press the button once and everything works as expected, I cannot focus those buttons anymore. When I move the mouse over and try a click a different control is focused instead and receives the click event. I cannot reproduce this problem in a simple test app and I do not understand what could cause such behavior.

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Added after 14 minutes:

The problem is caused by the Ubuntu 11.04 Unity Desktop, it does not reproduce on the classic Gnome Desktop.