Getting back to basics and practice are great suggestions.
But when I get in a slump I practice by the hour and not the game where possible. For most of the hour I just throw strikes. strikes after strikes after strikes. No, i'm not getting strikes, just throwing my strike ball and resetting the pinis that are left. Others might not agree with this practice and i can understand that.
But to get out of a slump you have to get your timing back and your muscle memory. If you throw a strike ball then pick the spare up and so on and so on, you don't get the muscle memory you want. Yes, if you're timing is off and your not throwing a good shot or using good form, you could get bad muscle memory. but shooting your spares won't help that.
Bring more then one ball and find a strike line that is easy for you to hit with a particular ball. Once you find that strike line with a ball, throw shot after shot through that strike line just trying to get the ball to the pocket strong. again, reset spares if you are on an hourly rate, if you're paying by the game, just throw another strike ball, don't worry about the other pins left on the lane.
Depending on your strike line and how long you throw the ball of course you're going to have to make the adjustments to stay in the pocket. make the adjustments.
If the lanes aren't flying, I use my strike ball for left side spares and plastic for right hand spares. bowlers do use their strike ball for all spares and others use their spare ball for all spares. Not saying what is right or wrong, just how i make spares. i make all left hand spares with my strike ball on my strike line. So continuing to throw strikes will help my spare game.
Work on push away, back swing, knee bend and release keeping in mind timing is the key. With someone with your skill level, you'll find where you struggle in your game if you repeat the approcah and release of the ball rather easily.
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Nicanor (Ten On The Deck)