Well, you could try using the "Silverlight-enabled WCF Service" template in VS2008, and comparing the differences? I expect that you need to use the basicHttpBinding and are using something more exotic.
For info, here is the web.config section for a default Silverlight/WCF service:
<system.serviceModel>
<behaviors>
<serviceBehaviors>
<behavior name="MySite.Service1Behavior">
<serviceMetadata httpGetEnabled="true" />
<serviceDebug includeExceptionDetailInFaults="false" />
</behavior>
</serviceBehaviors>
</behaviors>
<serviceHostingEnvironment aspNetCompatibilityEnabled="true" />
<services>
<service behaviorConfiguration="MySite.Service1Behavior" name="MySite.Service1">
<endpoint address="" binding="basicHttpBinding" contract="MySite.Service1" />
<endpoint address="mex" binding="mexHttpBinding" contract="IMetadataExchange" />
</service>
</services>
</system.serviceModel>
//For More Details Refer : Click Here
//I Hope This Will Solve Your Problem....