Map tips secret

[DonFreeman]

What is the secret to making the map tips work as in http://help.arcgis.com/en/webapi/silverlight/samples/start.htm#GraphicsMapTip ?

I can make it work with ESRI's map but when I substitute my own it will not work. Is there something special needed when setting up the service?
Thanks

[JenniferNery]

Check that Binding statements are correct, provide a Fallback value if you want to know whether the binding failed.

For example:
"{Binding [STATE_NAME], FallbackValue = 0}"

This will fail if the graphic Attributes do not include STATE_NAME as one of its fields.

[dotMorten_esri]

Note that you only get maptips on GraphicsLayer and FeatureLayer.