session trouble

On the recommendation of another, I have abandoned cookies, sort of, and gone for sessions instead. I've tried to write it so that when a user logs onto my site for the first time they input all their details a session is registered, which I (unoriginally) called $session from someone else's example, and place they username and password into $session. This works fine when I first set it up, and on all the pages that I check that it has been created it goes through fine, however, I offer those who already have accounts the ability to log on in the traditional means of username and password, but when I call upon the values stored in $session this time, they come out blank. WHy would a session suddenly lose the data inside it?