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I have been using the ArcGIS Silverlight API to create a mapping application. To provide some context, the application had to show a pipe network together with valves and other associated assets. The pipes were to be selectable in the interface so that pipe asset IDs could be used to drive queries and other processing.
In order to render the valves etc. I chose to use the ESRI FeatureLayer. I also used a FeatureLayer with a Mode of SelectionOnly for pipe selection.
One of the requirements of the system was that background imagery be used. This was created using an ArcGISDynamicMapServiceLayer. The feature layers and the background layer were taking their data from different ArcGIS services.
Although my code was using MVVM the scenario could be replicated in XAML like this (this is the map control XAML only with a number of layers omitted):

<esri:Map x:Name="AWMap">
    <esri:Map.Layers>
        <esri:ArcGISDynamicMapServiceLayer 
                 ID="BaseLayerStreets" 
                 Url="http://servername/ArcGIS/rest/services/projectname/backgroundservicename/MapServer" />
        
        <esri:FeatureLayer ID="Hydrants" 
            Url="http://servername/ArcGIS/rest/services/projectname/featureservicename/MapServer/0"
            Where="1 = 1"
            Mode="OnDemand"
            Renderer="{StaticResource ValveRenderer}">
            <esri:FeatureLayer.Clusterer>
                <esri:FlareClusterer 
                    FlareBackground="Red" 
                    FlareForeground="White" 
                    MaximumFlareCount="9" />
            </esri:FeatureLayer.Clusterer>
        </esri:FeatureLayer>
    
    </esri:Map.Layers>
</esri:Map>

The problem

The problem was that as soon as the background layer was included the feature layers simply didn’t render. Handling the feature LayerInitialized and InitializationFailed events showed that the feature layers were initialised and that no errors were reported.
So what was going on?

The solution

After hours of head-scratching I reread the ESRI documentation and this popped out:

“By default, the first layer with a valid spatial reference defines the spatial reference for the map. Dynamic ArcGIS Server map and image services as well as feature layers (FeatureLayer) will be reprojected to the map's spatial reference if necessary.” - http://help.arcgis.com/en/webapi/silverlight/help/index.html#/Creating_a_map/016600000011000000/

When I checked the metadata for the 2 services in the ArcGIS Services Directory I noticed that the Spatial Reference was different for the 2 services. So, I changed the XAML to something like this (note lines 2 to 8):

<esri:Map x:Name="AWMap">
    <esri:Map.Extent>
        <esri:Envelope XMin="111111" YMin="222222" XMax="333333" YMax="444444" >
            <esri:Envelope.SpatialReference>
                <esri:SpatialReference WKID="27700"/>
            </esri:Envelope.SpatialReference>
        </esri:Envelope>
    </esri:Map.Extent>
    <esri:Map.Layers>
        <esri:ArcGISDynamicMapServiceLayer 
                 ID="BaseLayerStreets" 
                 Url="http://servername/ArcGIS/rest/services/projectname/servicename/MapServer" />
        
        <esri:FeatureLayer ID="Hydrants" 
            Url="http://servername/ArcGIS/rest/services/projectname/servicename/MapServer/0"
            Where="1 = 1"
            Mode="OnDemand"
            Renderer="{StaticResource ValveRenderer}">
            <esri:FeatureLayer.Clusterer>
                <esri:FlareClusterer 
                    FlareBackground="Red" 
                    FlareForeground="White" 
                    MaximumFlareCount="9" />
            </esri:FeatureLayer.Clusterer>
        </esri:FeatureLayer>
    
    </esri:Map.Layers>
</esri:Map>

The result was that the missing feature layer magically appeared.
So, if you are having problems with missing feature layers check your spatial references. My guess is you should change the spatial references at the server to prevent re-projection on the client; performance may be better.