Tucking of the nose is like Kayty said, slow and patient work. The horse should not be ecpected to tuck to the point that its face is on or behind the vertical as this strains the muscles and impairs both sight and breathing as well as a stiff back and roughly muscled neck. You should be aiming for a smooth turnover and poll at the highest point of the arch as well as a nose slightly in front of the vertical to aid in proper breathing and sight. You should be aiming for proper collection and frame but also think about your horses anatomy, can you breathe as well when you tuck your chin to your neck???