Combobox Component Problem

I have created a Flash MX ComobBox from the Components. I have populated the list from an XML file. No selected item appears in the combobox. Using the drop-down the list is populated. Select from the list and nothing appears in the selected header. Press the drop-down and the selected item seemed to be selected. Use combo.getSelectedItem().data after a change and the correct selected option is shown.

The component is behind a timeline mask. Will this make a difference?