If you don't already have a free/loose backswing that can help a bit. Try just letting the ball swing free like a pendulum.
Also when practicing try to release the ball right at the end of your slide so you can get your body mtoion helping you. This will help to add the speed of your slide to the ball. Please DON'T confue this with putting your weight behind the ball. Watch some of the bowlers with higher speeds to see what I mean.
If you have anymore questions feel free to PM or post them on the forum.
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Bowler19
But be not afraid of greatness: some are born great, some achieve greatness, and some have greatness thrust upon them.-W. Shakespear
"Some are born great, some achieve greatness, other get it as a graduation present"
Robin Williams