Try putting a pole two to four strides from the wall, on an angle so that when you come across the diagonal on the right lead and go over it, the pole gives you a little lift to get your left lead and the wall encourages your horse to think about going the other way
I taught my 19 year old gelding flying lead changes by slowly getting the simple changes so that it was only one step and then right into another lead, and then used use poles on figure eights at a canter (with supporting needed* aids) and switching the lead over a pole. Hope this helps!
I always started with interrupted lead changes, in small figure-eights. I used a very strong heel to signal the lead change, and came to a complete halt in the "bar" of the circle. I'd advance to merely slowly to a trot, then finally, change at the lope in the figure 8. Once he's got that down, start in a straight line, with interrupted changes. Be sure to be consistent with that leg cue!