Study the part of the system that seems related to the bug to help you reproduce it

If you can’t reproduce a certain bug, study only that part of the system so that you can get it to fail. If it’s a problem with the session, study the class(es) dealing with handling of the session. See what methods there are – is the session getting cached in the db? Is it getting cleared from $_SESSION in an attempt to save space? Is there anything in there that looks like it might be related to your bug? Studying those particular aspects of the system will hopefully give you enough understanding so that you can make the bug reproducible.