How can I make the motion smoother?

I'am programming my first game for Ipad. I have a little problem on an animation. In this game I have a ball bouncing around the screen. I move the ball in this way

CGRect frameRect = ball.frame;  
frameRect.origin.x += ballMovement.x; 
frameRect.origin.y += ballMovement.y;
ball.frame=frameRect;

The ball moves but the animation is sometimes not very smooth... The strange thing is that while I was testing my app on my ipad I discovered that if I close the app and reopen it from the multitasking bar the ball moves way way better! The animation is smoother and fas开发者_如何转开发ter... Can somebody explain why is happening this thing?

Thank you!! Daniel from Italy

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If reopening the application makes it smooth, then most likely something has been cached the second time that was not cached the first time. There's nothing in the 4 lines of code you posted that could explain the lack of smoothness. Maybe posting more of the code would help.