Join up difficulties, opinions please?

the three signs of join up is when her inside ear is flicking towards you, her lowering her head and looking like she's chewing on the air. once you see her doing this, then you turn your back and let her come to you. in rare cases, some horses just won't join up. i've even heard many professionals say this. not saying that this is your mare, but don't get frustrated with her if she doesn't. some horses just don't have the instinct telling them to join up