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.