My back is a strong point for me, top to bottom. It's not Dorian Yates or Ronnie Coleman like, but it's my strong point, including the rhomboids. My posture while walking or sitting is perfect (I am former military), so it's just a sleeping thing.
As far as why I can't sleep on my back, it may have something to do with growing up wrestling. There isn't a wrestler in the world that likes to be on his back. In all seriousness, I can sleep on my back by force, but in the middle of the night when I wake up stiff, I always turn onto my side to get back to sleep. However, since I just had knee surgery a week ago, the only position I CAN sleep in without pain is on my back. So I'm learning......