I'm having a little problem with an a4j:support tag which is not firing.
When I select a country (placeCountry), the first a4j:support fires the onclick event and then rerenders the selectTown selectOneMenu, which then displays a town.
Now as the selectTown changes, it should fire the onchange event and rerender placesFound... but it does not.
When you reRender selectTown, you are changing the value on the server. It's not a client event, so nothing gets triggered on the client. You would need to actually change the value in selectTown for the Ajax request to be fired. Hope this helps.
You could try and play with oncomplete attribute and a4j;jsFunction to fire the next Ajax request.