There are three obvious reasons for action method not to be invoked:
Ajax request is not fired
Check if the ajax request is fired by using any http listener, firebug plugin for firefox or <a4j:log>
Validation error on the server
If there is an ajax request to the server and your action method is not invoked, this is propably because of validation error on the server.
Put the following:
<a4j:outputPanel ajaxRendered="true"> <h:messages ></h:messages> <a4j:outputPanel>
The rendered attribute refers to a request scope bean