We are shooting in the dark here. If you send us a sample project it would be very helpful to investigate the problem. Also, you could refer to the Valeri Hristov's blog about RadComboBox and WCF service - http://blogs.telerik.com/valerihristov/posts/08-12-04/auto-complete_combobox_with_wcf_service_and_radcontrols_for_silverlight.aspx
I hope this helps.
All the best,
the Telerik team
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