ValueChange Event not fireing

I have Class which extends a Form and it implements Property.ValueChangeListenter

like this:
public class AddressForm extends Form implements ClickListener, Property.ValueChangeListener
_zipCode = (TextField) getField(“zipcode”);

and the following valueChange method

public void valueChange(Property.ValueChangeEvent event) {
    Property property = event.getProperty();
    if(property == _zipCode) {

Everything works fine until I call setFormFieldFactory(new AddressFieldFactory()); method
After I call that method the valueChange event never fires again even if I don’t change the zipcode field in the AddressFieldFactory().

What am I doing wrong?

Thank You