My first attempt at girl-on-top was awkward and didn't work at all, but then I figured out how to position myself. I had been trying to get on top of my partner like he would get on top of me, but our anatomy just didn't line up. Finally, I straddled his hips as if I were just going to sit down and that did the trick. Sometimes his bits need a bit of manual help to get inside, other times I can just slide down him with no problem. I think it might be a little easier to achieve penetration if I lean over him slightly at first, kind of like if I was going to go down on all fours. After he's in, I can sit back up, lean back, or do whatever.

Hope that helps. If you edit your post with some more information as to what exactly the problem is, maybe you could get some more specific advice.