Have you tried running this under Firebug and watching the Net tab? Is anything failing, particularly under the XHR subtab?
No failures. I think this is the same issue as explained here -- http://www.silverstripe.org/data-model-questions/show/252763#post252763
There is a bug filed related to it (http://open.silverstripe.com/ticket/3428) but no perfect solution has been presented yet that I can find.