Our affiliate cookies have a life of 30 days. It means that you will get credit for anything that the user buys for 30 days from the moment they click on your link. There are a few situations when this doesn't happen.
Firstly, if a user clicks on your affiliate link and comes to the site, but then closes it and clicks on another affiliate link, the most recent link will be the affiliate ID that gets credit for any sales. Secondly, if a user deletes the cookies on their browser, which some people do for privacy reasons, their actions will not be counted for your credit.
These situations are rare and unfortunately just a reality of being an affiliate, it's standard for every affiliate program.