Well, I was puzzled but I am no longer. I had the limbs bending fairly evenly, then suddenly the right (top) limb was bending much less. I was examining the backing and found a crack in the backing on the left (bottom) limb. Dad gummit, or words to that effect. I might just steam the backing off of this, slap some rawhide and snakeskin on it to make it a kids' bow (the neighbor down the street wants me to make one for his kid for christmas), and get on with my life.
I hate to do this, gentlemen, but I think I'm going to have to declare dogbane a failure as bow backing, at least for me. I could try again but I just don't know how to do it any better than I did.
As WC Field said, "If at first you don't succeed, try, try again. Then quit. There's no use being a $*#& fool about it."