I have had the same problem in the past and still do on occasion and it is usually due to MY positioning. Make sure the the triangle you make which is formed by the horses body from head to tail, the lunge line to you which forms the point and the whip which makes the third line of the triangle are at the proper angle. You dont want your triangle to look like a perfect 45 degree angle on each point you want it more like a 90 degree where the whip and butt meet. Basically, make sure that your body is not inline with his barrel or back, you want it to be either directly inline with his butt or just slightly behind it. So your triangle is at a 90 degree angle or just a little obtuse. You need to be driving them from behind in order to get them to propel forward at a faster pace. If you are at their shoulder you are going to tell them to turn or stop, if you are at their side then you will tell them to slow down and if you are behind then you are telling the to move ahead. Also, if you are at incorrectly at their side and not behind or parallel with their butt then when you flick the whip you could run into the possibility of disengaging the hindquarters and causing thier butt to swing out and have the horse turn to you and try to stop.