Newbie btn inside mc loses functionality?

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I have a movie clip that contains a button the buttons are created dynamically using xml. The button has a rollover change colour effect. When the movie is run the buttons change colour as expected when the mouse rolls over them. However as soon as I add code to a button to perform an action the button loses the rollover effect.

Why is this??

Is it something to do with event capturing and propagation?

How is it possible to overcome this??