Threads are marked resolved when the thread starter marks them as resolved. There is no other reason beyond that.
And so far, clearing cookies works on all cases I've seen. If you have a case where it doesn't, and can debug why, we'd be glad to hear from you. I have been unable to duplicate any problem along these lines that clearing cookies will not fix.
Also, if one browser works and another doesn't, then clearing cookies on the one that does not work will *absolutely* fix your problem.
Firefox: Tools->Clear Private Data. Check cookies, hit clear button.
IE7: Tools->Delete Browsing History->Delete Cookies button.