Why does a button does look pressed when clicked over?

I am just starting with wxPython but already made a complete app in less than a
week. wxPython is *great*.
Thanks for the all the people that designed it and contribute!!

Question:

Is there any attribute to a wxButton or wxBitmapButton that can affect the
button behaviour and prevent it to look "pushed in" when the user left click on
it?

My app had button from day one and they looked fine. Then just today I realized
that the buttons dont push down anymore. The events are generated and the
methods are called, but the screen representation of the button does not change
when the user "presses" on the button.

I added a 'new' button to my app, and the new button works fine. Any idea?

I am using: wxPython 2.4.0.7, Python 2.2. Tried the app on Windows NT 4 SP6,
Windows 2000.

Many thanks,

Pierre