The most obvious sign that a horse has not accepted the contact is resistance. This can be displayed through snatching, falling behind, grabbing and hollowing.
Snatching
Horses usually snatch at the reins when you grip them too tightly. This can usually be fixed by gently sponging to encourage softness and suppleness. Don’t confuse this with sawing (pulling the bit from side to side).
Falling behind
This means that the horse is curling back to be behind the bit in order to evade the contact. If he does this he will be behind the vertical and you will lose the connection. To fix this, use more leg to push him forward into the contact. Make sure that your reins are not so short that he is being forced behind the vertical.
Grabbing
A horse might respond to the contact by grabbing the reins from your hand and pulling his head down. In this case, it’s important not to let this become a habit. Set your hands where you would ideally like him to be and hold the reins firmly when he tries to grab. By doing this, you allow him to just fight against himself and he will learn that the answer is to keep his head at the position in which you stay soft, until you release and allow him to hold his head where he wishes.
Hollowing
If a horse responds to the contact by hollowing, work him in a long and low frame. It will help to develop his topline muscles, which will help him hold himself in a frame.