It is very hard for someone not watching you bowl to say what is happening with your tourney scoring. Aside from different conditions than what you normally bowl on, there are other variables at play here.
You are probably the best judge of your game. Sit back and answer these questions to yourself about your tourney performances.
What is happening in terms of ball reaction from games 1-2 to 3-4 to 5-6 that is dropping your carry percentage as you go on? Could be a failure to properly compensate for lane transitions?
Are you still hitting your mark with a consistent release? (speed and revs staying consistent?)? Could be mental lapses. Try to concentrate on 1 shot at a time, and not look at the overall score.
Are you tiring out? Could be physical, changing your practice habits to bowl more games in each set should help you get through the physical stress.
How do you convert your spares? Improving your spare conversion rates will help keep your scores up when you're not making strikes consistently.
There are many good tips listed from the people here. But, you have to honestly assess your performances and figure out what exactly is causing your scores to drop. Then, you will know what is causing the problem and can make the appropriate changes to fix the problem.
Good Luck on your Mission.
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"Beer is proof that God loves us and wants us to be happy." Ben Franklin
Enjoy,
Bob