Set Focus

Hi all,
I have a small problem with setting the focus of text fields in a form. I have the following code on the _root timeline:

stop();
Selection.setFocus(_root.form.first_name);
myListener = new Object();
myListener.onKeyDown = function() {
if (Key.isDown(Key.TAB)) {
Selection.setFocus(_root.form.last_name);
}
};

The text fields are held in a movie clip called ‘form’. It sets the focus to ‘first_name’ but will not then focus on ‘last_name’ when I hit TAB.
I have about 5 text fields to go through.

Any ideas why it won’t focus?

Cheers guys,
Pat