Louise, have you tried logging in using the Login radio button that's alongside the others ie Home, Profile etc? If you login using that button, there's the option to
Always stay logged in: check that box and that should solve your problem. The 'Forever' option that you have been using gets over-ridden by other options we have set, so no matter if you select it, your session will get timed out if you are inactive for 60mins.
HTH
