Tom, I think you would agree that the cause can be a number of things:
1. conformation of the horse and its left/right bias,
2. inconsistent/ineffective combination of the aids being given by the rider,
3. poor rider body positioning,
4. tack that is not suited to the particular horse at that point in time (incorrect/uncomfortable bit or irregular saddle fitting for example)
5. over development of the horse on one side due to the training not addressing the weaker/blocked side and training being conducted on the good/easier side predominantly
6. lack of understanding by the rider as to the aid combination and the expected outcome required, &
7. it could be that the horse is not developed enough mentally or physically to complete the moves being requested
Some food for thought and I will be interested in your reply :)
