You may want to check for the value of maxItemsInObjectGraph
attribute in the service behaviour.
A sample would look like this:
<serviceBehaviors>
<behavior>
<dataContractSerializer maxItemsInObjectGraph="<your value>" />
</behavior>
</serviceBehaviors>
You may want to check this question over the Stackoverflow:
how-to-fix-maxitemsinobjectgraph-errorhow-to-fix-maxitemsinobjectgraph-error