I am a jumper rider but I am coming to the wise ones of dressage land for some advice.
I have a stocky 16.3 hand ISH. He has good conformation and is built neither uphill or downhill. His right hind is weaker than his left. He can be lazy but also dull and spooky at the same time (I've heard that this is an Irish thing!) He bends easily around my right leg but wants to fall in on the left.
My problem is this. I need to get him to step away from my leg better. I know that I am causing the problem as others who have ridden him have no problems. Our diving around the corners (especially to the left after a fence) has become problematic. Today I decided to go back to walk work and really focus on getting a good, correct leg yield-back to basics as it were. With light contact, on a straight line as soon as I add leg behind the girth his shoulders fly out and his haunches don't move much. If I try it with adding hand to block the shoulders we lose forward (and as we have a problem with forward in the turns on course I want to avoid this). If I get after after him with either spur or dressage whip, he gets anxious and then bounces off my leg (I want to avoid this as sometimes on course he will get quite strong or bolt after a fence).
First, make sure you can move his hind end around when you are on the ground. If he will not cross his hind leg over when you apply pressure where your legs would be on the ground , there is little chance he will respond with you on board.
Second, when under saddle, make sure you apply leg pressure at the right time, meaning, the moment when his inside hind leg is leaving the ground. If you apply pressure when his inside hind leg is planted in the ground, there is no chance he will do a leg yield. Once he responds, return your leg to neutral (no pressure, just resting there).
When I do leg yield, I actually apply my inside leg behind the girth, not at the girth. My outside leg is also behind the girth to guard against him swinging his hind end out.
Then, I think of leg yield as two-step process. step 1, inside leg, step 2, outside hand. So, you apply inside leg to ask him to move over, then, you block his shoulder from escaping by using outside hand. In step 2, think of bringing his shoulder back to left (if you are leg yielding from left to right), then repeat.
[Edit] "think of" bringing his shoulder back is different from "actually" bringing his shoulder back. That is a feeling, not an action. If his outside shoulder is escaping, thinking to move forward/side way more will make it worse, hence, the "thinking" part of bringing his outside shoulder back. What you want is to ask his outside shoulder to wait for his inside hind end. So when you "think" of bringing his outside shoulder back, in reality, you actually tells him to "stop your outside shoulder where it is for now, until your hind end catches up". Hope this is clearer..
If your horse is not answering your leg, you might want to think first of turn on the forehand, when you have that nailed stride for stride, go to turn on the haunches, allowing a little more forward than a pirouette would require.
Then try your leg yield. First go straight! Then keeping your outside rein steady, with a slight inside rein flexion (you want to see no more than the curve of his inside eye), apply pressure with your inside leg, and move your body in the direction you wish to go, the outside leg is a guarding leg which in conjunction with the outside rein will keep him from dropping on and leading with the outside shoulder. Think forward, and over! A slight half halt, in between the forward and over, will give him the fraction of a second required to cross his inside leg over his outside leg. If he drops on the outside shoulder, or starts moving diagonally, the outside leg must correct him-or you can go straight for one or two strides, and then try again. Just remember, when your body and hips move sideways, they stay there. Do not rock back. It's a one way street.
Some riders change their horse, they change their saddle, they change their teacher; they never change themselves.
Leg at girth. Hold outside rein, bend a little bit with the other rein, not enough to stop him, just to keep him straight, bend, but just near the throat, not along the whole neck. Leg at the girth, praise when the horse moves over.
If the horse is not reacting to one leg, I'm not so sure the answer is trying to do leg yields, in which he also doesn't respond to your leg. Probably the key in both situations is that when you put one leg on, he moves over, or he gets a quick clear correction, a tap with the whip.