Probably it´s because you click in all ads ,and then you come back later to see if there is new ads.
But when you come later , your cookie session was expired.
You only need to logout , and login again if you come back later on.
Actualy, everytime that you leave out , you should logout.
Maybe there is some glitch , when a member come back later.
Our site use a secure conection ( https ) even on the ads page , wich can be the cause of that small issue.
We need to live with that.