A lot of merchants use cookies for tracking your referrals. As a majority of customers don’t purchase on the first visit, cookies allow for potential referrals to be tagged with your ID so that if they do purchase later, you’ll get the
credit for the sale.
The durations for the cookies will vary from merchant to merchant. Some last as short as a single session, while others can last for years. If the visitor flushes their cookies on a regular basis, has cookie blocking software, or the program from the merchant isn’t operating correctly, there really isn’t anything you can do.