Cutarug, I'm thinking that jester is right and it's falling out of the loop due to insufficient speed. What's happening is that as you go over the top it stops pulling outward and the G load on the model goes negative. Like inverted flying negative. But not fast enough so it stalls and flicks out of the loop. Try a little more speed/longer dive and use a little extra up as it slows down to see if it makes it around.