We have picked up a bug. On some mobile browsers this sub menu is not displaying.
Our developer has been contacted, however since we are coming into the weekend it might be Monday before it is resolved.
As a containment action, I have moved "Forums" and "My Courses" into the header menu bar where they are visible in desktop and mobile browsers (that I have tested).
Please let me know via reply below if you have issues using this method to access the site.