Hello Stephen,
As I mentioned in the support ticket you have opened for the same issue by default the ASP.NET AJAX Framework cancels the ongoing ajax request if you try to initiate another one prior to receiving the response from the first one. See the help topic below:
http://www.telerik.com/help/aspnet-ajax/ajax-request-queuesize.html
Also I have prepared a small video which demonstrates exactly the same behavior:
http://screencast.com/t/0TJ0BUgGt
Regards,
Maria Ilieva
Telerik
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