The one thing that usually causes a problem such as you are experiencing is failure to check off the "Remember me" box when you login. If you do check it off, you will still be logged off after the usual idle time period (20minutes), but you will also be logged in automatically when you return. Often a long post will take more than 20 min. to compose, or you get sidetracked for longer than that.
So, the next time you login check off the "Remember me" box and see if that fixes your problem.