At least on first use of the system, Java verifies the certificate of the applet and asks for a “Trust confirmation”. The windows related to this confirmation, sometimes, appear behind the browser window and cannot be seen. In this case, move the browser window to the side (do not minimize them) and check if any confirmation window can be seen.

If this is not the problem, we recommend that you clean the cache of your browser, ideally close it, re-open it and try again. This usually solves the problem.