"@Lougle the puppy classes have us sort of slowly moving a treat down from nose to paws to sort of lure them down to the lie down position and treating as you go. The elbows out/neck stretched thing is a mid way version of lie down."
Does she know how to lie down? Has she successfully got to the point of all four paws on the floor, resting on her hips and her elbows, with her bum on the floor? If she has, then I wouldn't be treating her for anything less. She should, at the very least, be able to follow the treat all the way down to the ground. I would crouch in front of her, bring the treat down low under her nose, and then move your hand down to the floor. Hold the treat and don't give in to any pawing, any sniffing the treat and trying to nibble it out of your hand, nothing. The only time you release that treat is if she is actually lying down. Then big praise, whoops, awesome dog you are! She has definitely learned that it's good enough to crouch.
It's a bit like when you do a recall. If the dog learns that running to you, grabbing the treat, then running away is ok, then they don't actually come back, they just smash and grab. You have to move your hand towards you as they come, so that they have to come right close to you to get the treat. Then treat and, ideally, send them off with an 'Ok' so that they don't just get put back on a lead if they come to you.
It's hard work but you will get there. We all will.