Okay, so you complete an offer. Let's say a Tickle test.
Now...After you put in your billing information, and purchase the test results, let the confirmation page load. Keep that window open for about 10 - 15 minutes (that's what I do). I do this because sometimes, the microscopic image on the bottom of the page doesn't load right away, and that causes people not to credit. That image, is part of the entire crediting process. You can't see it, because it's only 1px by 1px.
You should then credit after leaving the window open. You can't close it, or go to another site in that same window, but you can open another window.
There was one time where I did an offer, and the image never loaded. So I was screwed over. But it happens. Just leave the window open for 10 - 15 minutes, and you should be fine.
This always works with me as well. This should be added into TWG's thread.