(“The usual suspects are. . . “)
Aside from a cadre of hyper classical troglodytes, almost everyone accepts leg yielding these days. Riders dutifully practice the First Level test patterns— yielding from centerline out to the track and from the track inward. I also see lots of spiral leg yielding outward from a 10 to a 20 meter circle. What I commend to you is to become proficient at leg yielding in another pattern: yielding along the track tail to the wall, essentially making a shoulder in without the bend.
Ask yourself what goes wrong in shoulder in most often. The usual suspects are the neck overbent to the inside, the horse restricted in front and losing energy, and the horse falling over his outside shoulder and leaning on the outside hand. These all result from the rider’s inability to control the horse’s lateral balance.
As a remedy try practicing leg yielding along the rail with an absolutely straight neck as it comes out of your horse’s shoulders. Only position him to the inside at the poll.
Furthermore, don’t make the angle by taking your inside leg behind the girth. Keep your legs in the same position they would be in for a shoulder in. By managing your outside aids you can regulate or limit the bend as you choose. Create a straight horse (on an angle) who is willing to hold himself up as he moves along the track.
When you can produce such a balance, superimpose bending through the length of the horse’s body onto it. A good way is by leg yielding as described down the long side, executing a 10 meter circle, and carrying that bend (and balance) as you proceed on three tracks to the next corner.
You will also find that later on when you want to produce a renvers, it will be much easier to change the bend without pulling your horse’s head to the side and to put him into the new outside rein.