ArcGIS Online Basemaps not visible after adding in maps from server

Hi,
I'm using Flexviewer 3 and ArcGIS 10.1. When I create a new application I can view all the basemaps from ArcGIS online fine, but when I add my own maps in and click on any of the ArcGIS online maps, it just shows a blank screen.
Does anyone have an idea why this is, is there something blocking it that I have to allow access somewhere?
I've the firewall turned off on the server and I still have this problem.

Thanks

Rachel