if the problem is that your shots goes too weak and centered, you have to keep the button pressed all the time until the ball is off the racket, if you stop before it's off the racket, your ball will go to the center and very weak, like a safe shot(no button pressed at all).
If it's not exactly that, do you play with the danger zone preview?, if not, turn it on, and see that you will have 2 green zones, one for forehand and one for backhand...always try to be positioned at either of those zones as soon as you can, if you're well positioned earlier, you should be able, if you do what I said above, to hit as hard as your power and top spin skills lets you. To maximize the power, try 0 top spin and 100 at power, that way you'll have a max speed of 150km/h on a regular basis and even more in some shots, but it will be riskier.
Hope I helped
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