(I assume you're talking about slapping, not popping.) In my experience, slapping had a particularly steep learning curve, whereby it seemed almost impossible at first, but one day it just suddenly started working. I remember that at first I tried very hard to keep my thumb straight and to hit precisely on the joint of the thumb, but in reality, neither of those are strictly necessary for the proper accuracy and tone. In fact, I find that I really only developed a feel for slapping when I started hitting the string somewhere between the joint and the tip of the thumb. Like JGbassman said, I think it's essential to just keep trying slightly different positions and attacks, and eventually you'll get a feel for it.