As for your own server, it could be a few things. Browsers will complain if you access an http imagery server when the web page itself is accessed over https. It could also be a CORS problem. See the Cross Origin Resource Sharing section of the Imagery Layers Tutorial. Use the console in your browser’s debugger to see if you’re getting any exceptions or error messages. Sandcastle has a console, but it unfortunately can’t include CORS-related error messages.