Ok so me and my boyfriend have both been riding since like 2.. and he has a 10 year old quarter horse gelding.. this horse has the perfect lope EXCEPT when you ask him to lope off to the left he will pick up his sholder like he is going to take it, then at the last second he drop it and picks up the right lead

.. we have done everything we can possible think of to help him pick it up.. and he will pick it up when we lunge him.. we really need help! We are open to any suggestion!