Yeah, don't exercise the bow until you like the tiller, but you are way under your target at 21 lbs/15" If you can pull it 15", fer heck sakes, get that long string off it and get a real string on! The long string is only for the first few inches.
If you want a 40 lb b ow, just don't pull it more than either: enough bend to see what needs correction, OR more than final draw weight.
It sounds like it is bending all at the inner limbs. You need to spread the strain as much as possible. If it bends mostly at the inner limb, and not the midlimb, it will take set at the inner limb. That is the worst place to have set, because a little there affects the tips a lot. So, leave the inner limbs alone, and chase that inner limb bend out along the limbs to the middle section. The only place you want stiffness is the tips, maybe a thumb to pinkie spread length on a 70" bow.
