This sounds very much like She is just not quite ready.Usually a bitch will do this type of behaviour prior to being spot on.Some bitches will never stand even on the day,but usually if you or someone whom she knows holds the bitch at the head end,it may encourage her to allow the dog to mount.Sometimes you will have to put a hand under her stomach also to stop her turning away etc.
I have had bitchs do this and I usually hold them at the head and rub my hands up and down her back,so to speak,as the dog is at her rear and it helps to pacify her to allow the dog to set paws on her.
It is d