First off, are you sure that it's your ball speed? Does your house have speed radars in them or are you guessing?
If that's what it is, make sure that you have a free armswing, and make sure you start the ball from the same spot every time. Starting at your waist and shoulders will have two different amounts of speed, so make sure you're doing that the same. Also, make sure you aren't forcing it or changing your routine on some shots.
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- Andy