Leg yield in different ponies

*WINK*

Ok, I'm confused. Leg yield is not natural, I swear. Jargle, my friend'd horse, is at a massive 17hh, but can leg yield around the arena from one side to another, hardly moving forward. For all the love in the world, I can't make Chocolat, my other friend's pony, move even one step sideways, even though he has been trained super well in all aspects of equitation and can do it in a second.

So what do I do?

I think I'm giving the wrong aids. Maybe the aids differ from horse to horse. Inside leg and outside rein contact, slow walk/trot/canter and inside rein allowing movement. RIGHT!?

Do some ponies just hate me?
It seems rather likely :)