This might help. If your reins are of equal length, and you have the same amout of weight in both of them, you do not have enough bend. Logic is, if the horse is flexed properly to the inside, your inside rein will have more slack in it than the outside out of sheer physics. Tiny's right. It's all in the bend. Once you get the correct amout, the hose un-kinks and you get a connected horse.
When you do that circle work, if you still cannot get your horse to soften at the jaw, try practically leg yielding him around the circle. It'll be overkill, but will give you the result and feeling you need to work on.