This is why the affiliate relies on cookies. A cookie is simply a small file consisting of a text-only string that gets entered into the memory of your browser. This string contains the domain, path, lifetime, and value of a variable that is set by the site. When users return ten days or thirty days later to make a purchase, the sale is credited to an affiliate. This is not a flawless system.
Users can delete cookies manually.
They can purchase software to do it, and there are software constraints. For example, Netscape limits your total cookie count to 300. If you exceed this, the browser will discard your least-used cookies to make room for new ones. All pose problems for the affiliate, but for the time being cookies will be the de facto standard until a new method takes its place. With this knowledge in hand take a good look at your site.