Ted,
A couple ideas for you. First, try to get your target up at shoulder level. If you look at your bow arm to spine angle, your bow arm is angled down. When you practice, try to keep your bow arm to spine angle at 90 degrees. This is very important for a consistent draw length and for strength too.
Second, watch your string arm and elbow as you draw. See how all the motion is in an upward direction? I can see no transfer to the back muscles. That upward motion is done with relatively weak muscles on the top of the shoulder and the arms; this equals no back use.
Don't get me wrong, there is nothing wrong with a "swing draw" it is just that that technique makes it harder to engage the back -- not impossible, just harder. At the end of the draw an effort needs to be made to transfer to the back.
Gentle enough?
Arne