Princess Debut's trace-pattern-to-music system is extremely unclear about how well you're doing. It's quite easy to intentionally-or-accidentally slide your stylus WAY off the path in completely the wrong direction and back on quickly and still score a 'Great!' It is equally easy to score an 'OK' or even a 'Bad' with absolutely no clue of what you did wrong. If you do get a 'Bad' there will be a split second at the END where the ball-trail displays redmarks in the vague section of the trail where you did something wrong, but it doesn't indicate what you actually DID, so if as far as you know you were right, you won't be any more enlightened.
If they left a trail behind where you actually dragged your stylus, then at least you could see what you'd done and figure out how to improve. (Particle-stars spawn around your stylus as you move, but they don't stay there.)
Also, this is a tough game to play when you have a dreadful cold and your eyes and nose may suddenly rebel!