After another ball machine session, I discovered that most shots that I miss are the result of my posture breaking down. When I consciously kept my back straight on every shot, even when having to bend at the knees to reach a low bouncing ball, my consistency was vastly improved. Any time I started to slouch, I was likely to net the stroke. This was especially true on my two-handed backhand. Even when practicing volleys, the same seemed to be true. A coach might have picked this up years ago, I'm learning through trial and error.