I started playing 2 months ago and the past 2 weeks I have been practicing everyday, either against the wall or with friends. I improved a lot but still I find out that I can't hit the ball using the sweet spot on my racquet. When it doesn't the racquet will vibrate a lot and the ball doesn't have power. How do you practice making sure that at least 90% of the time you hit the ball with the sweet spot? I try very hard the past week but doesn't seem to get any more consistent.