This can sometimes be caused by out of date browser.
Please make sure your browser version is up to date to the latest version. Most browsers will update automatically (or have an option to do so).
To check for pending updates, view Chrome instructions or Firefox instruction.
Once it has finished updating the browser to the newest version, close your browser window and re-open. If this does not solve the problem, please ensure your device, and network connection meet the Bongo basic requirements.