How do I debug WCF Duplex server callbacks vanishing on a silverlight client?

So, I have a duplex server housed in a service that provides webservices to a Silverlight 4 control. Most of the time everything works -- but one specific callback which should retrieve a collection, never seems to be consumed by the client. I can step through the server code and see the issuance of the response, and, if I watch in Fiddler, I can see my data sent in a transmission that's identic开发者_运维技巧al to those that work. On the client, however, my break point for the event handler is just never tripped -- I've verified to the last point I can in the client that the callback is still set.

What I can do to debug further?

EDIT

I figure I should also specify that I'm not seeing any first chance exceptions generated in the output window in visual studio on the client or the server that might be getting handled, and hiding the error.

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Are you having this issue?

http://blogs.msdn.com/b/silverlightws/archive/2010/12/15/pollingduplex-using-multiplemessagesperpoll-issue-in-latest-sl4-gdrs.aspx