Chances are you aren't shifting your weight enough or changing his(?) bend. When you canter yo have a certain way of getting your horse on the correct lead. You want to duplicate this when asking for the opposite lead form the simple change. If you can't get it on the diagonal or straightaway, work the change in the corner. Canter across the diagonal, come down to a trot three or four steps prior to the corner, lift your inside hand slightly to lift that, now new, inside shoulder and shift your weight slightly to the outside, change the bend and ask for the new lead in the corner. It helps gives the horse a visual clue as to which lead he/she needs. Once you have that down and are pretty confident in how you ask for the lead and the correct position, try the change in the center across the diagonal. Simple changes aren't as easy to do as people think and a horse who is unbalanced won't get the change. The important part is to set it up...horses just learning a simple change may need nine or ten trot strides to balance in preparation for the switch.