Turning the horse to the outside will unbalance the horse and "throw" him onto the inside lead. As you found out, it doesn't always work... I was also taught that by a couple of trainers, but have since learned much more effective, and consistent, ways.
Usually, inability to take a lead is due to an unbalanced horse. To help strengthen and balance the horse, you should work on lots of trot poles and pattern work. Working trot on a 20 meter circle as well, working from true bend to counter bend, does wonders. When you counter bend, hold the horse in a full bend (nose to tail) until the orse relaxes and balances, then slowly take him back to true bend. Relax there for a few strides, then back to counter bend.
The balance of the rider can also effect the horse. Be sure YOU are balanced in the saddle. When you go to pick up the lead, LOOK UP, breath, keep a soft back and shoulers, look around your turn, let your inside seat bone creep forward just a bit, lift up your inside rein just a bit, and ask with your outside leg. Time your cue to when the horse is putting down his outside hind (inside fore) leg. Have someone call it on the ground so you get the feel at the trot.
My mare had a similar problem and I discovered that we BOTH were causing the problem. She was not as balanced to the right and I colapsed my right shoulder (I had NO clue! Lol). Once we got both "fixed", her right lead was much more consistent. To increase her balance, we also worked on true bend-counter bend on a 20 meter circle at the canter. After we did that for a while, her leads were both perfect!
Good luck, and I do agree that this trainer may not be worth spending your money on