It is almost certainly your body language has the horse confused. I know I said this on your other thread, but lunging, either on a line or in a round pen, is not as easy as you would think. You need someone to show you how to do it. Seriously. It's worth taking a lesson just in lunging, if you can. Or at least see if some older, more experienced person will help.
I know you want to just go out there and do it on your own, and you can. But you may turn it into a really frustrating experience for both you and the gelding. I doubt he is enjoying being whacked on the butt.