Angry birds uses and electromagnetic solenoid to fire the ball at the game screen. When the solenoid fires the washer on the end of the plunger makes contact with the ball launching it at the screen. If the plunger washer has loosened or fallen off it will cause the ball to shoot crooked. When the ball is loaded into the shooter the ball should rest flat against the solenoid plunger washer, if the cup that the ball sits in has become loose or damaged it will cause the ball to sit at an angle causing the shot to be crooked.
The balls on the game are made of a compressed foam, if the foam inside the ball has started to breakdown when the solenoid fires it will absorb the impact causing the shot to be weak. If you were to squeeze the balls they should feel firm if they are squishy you should replace them, the ICE part number for the balls is AB3024X. When replacing the balls make sure to perform a calibration of the spotlight and the sling shot and NEVER use more that 2 balls in the game.