I would make sure you have endpoint behaviour configurations in the client and server with the maxItemsInObjectGraph property set
<endpointbehaviors>
<behavior name="endpointBehavior">
<datacontractserializer maxitemsinobjectgraph="2147483647" />
</behavior>
</endpointbehaviors>
Make sure each endpoint binding uses this behaviour like:
<endpoint behaviorconfiguration="endpointBehavior" ....=""></endpoint>
secondly I would check your client http request rate:
<httpruntime executiontimeout="90" maxrequestlength="20000" usefullyqualifiedredirecturl="false" requestlengthdiskthreshold="8192" />