If by basemap you are referring to your own map service, you'll need to provide a token in order for it to appear.Here's some documentation on how to acquire a token for use.http://resources.arcgis.com/en/help/main/10.1/index.html#/Acquiring_ArcGIS_tokens/0154000005rp000000/If you're referring to tiled map service like Bing Maps or ArcGIS Online, this could be another reason. You may want to use a tool like Fiddler or HTTPFox to determine what the response/error is.Hope this helps you get started.
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