Something that has really cemented in me over the last year is that if a horse is not showing progress in their training program, it usually means the question you are asking them is too confusing, or they physically or mentally aren't able to do what you are asking, without a degree of discomfort.
For that reason, I like to regularly run my hands over my horses bodies to ensure they don't have any tight or sore areas. If they do have tight areas, I will have them checked by a chiropractor, contact care worker or vet.