Giving the reins when the horse drops its head isn't much of a reward. Suddenly the contact is gone and the horse throws its head.
A horse should be very established in the contact before you start giving away the reins. I got very, VERY told off by my coach, who rides at an international level, for giving away my inside rein by less then 2cm, to scratch a green horse's neck when he came through. I got told off, because at this stage in the horse's training, it is not yet balanced and confident enough in the contact yet, to support itself when you give a rein away and change your balance. The best reward you can give, is to cease applying the aid.
For instance, if you have been asking for the horse to step laterally on the circle, you'll have your inside leg on the girth and so on depending on what movement you're asking for. As soon as you get the requested movement, you don't completely abandon the horse by dropping your contact. You MAINTAIN the contact and the movement, without increasing pressure by way of applying an aid. Just leave the leg quietly at the girth, keep the contact, and the horse is happy.
Kayty, I just had a crystallizing "OH!" moment. I feel so stupid for not realizing it before. I've been asking for the contact then when I get it, stop asking and a couple strides later give a little scratch on the neck and then getting annoyed when an hour later he's still not being so consistent in contact lol #feelsabitdumb