That probably comes from the traffic you're getting. You can ask your CPA account manager/affiliate manager (if they have one) to see what the best converting offers are. Those usually do well, and you can even request more information about those at your affiliate manager.
You can try out maxbounty if you want, I never really succeed at affiliate marketing but I heard they are the best (and if you're lucky you get a good contact from it). Goodluck on your ventures and if you need help let me know