Progress in Taiji (and there must be progress otherwise you're not travelling a path, you're just standing still) is a slow (think decades) process of getting closer to the ground. Sinking, sinking, sinking, which requires stronger legs, more flexible hips, more open groin. When I encounter old students who have kept up their daily practice but without a teacher to spur them on (fire their spirit), this is what they lack: their Taiji is all upper body with no root.