Many, many riders focus on the head and neck when schooling. At Elementary levels upwards, a greater element of collection is needed, but a lot of riders forget that their is an entire horse to collect. Their version of "collection", not that it really is that at all, comes from bringing the head and neck up and in. Meanwhile, the back end trails out behind the horse, who hollows their back and drops onto the forehand.
Horse can't bend? Shoulder-in. Horse not bringing the inside hind under? Shoulder-in. Horse heavy on the outside shoulder? Shoulder-in. Pulling in transitions instead of pushing? Ride them in shoulder-in. Needs the wall for balance? Shoulder-in. Not enough expression or suspension in the paces? Shoulder-in.
I am a dressage trainer and general equestrian coach in Surrey, Sussex, and Berkshire. I teach dressage lessons, and hold a range of riding and equestrian clinics around the UK, and use my blog to share horse training tips, advice, and resources.