It looks like perhaps the spherical falloff zone is too small. Open the joint editor (you can do this in the pose room), select the arm, select "bend" (or whichever direction is giving you this problem), then select the green circle that appears and scale it up a bit. You may have to do the same to the red circle. Everything inside the green circle moves. Everything outside the red circle doesn't. The stuff between the two kinda moves, depending where it is in relation to the two.