Soggy Cookies
Many merchants use cookies to track your referrals. This allows those customers who do not make a purchase on their initial visit to be tagged with your ID, enabling you to receive the credit should they buy at a later date.
The lifespan of the cookies vary from one merchant to another, some only lasting for a single session whilst others go on and on for years.
Unfortunately for you, if the visitor makes a habit of flushing their cookies regularly, has software to block cookies, or if the merchants program is not functioning properly there is very little you can do about the situation.