A ball catch uses a spring loaded ball (often made of brass) mounted on a door to catch with an interfacing hemispherical indentation on the door frame. This allows for the door to be opened and closed without turning a knob but to still remain securely closed when in the closed position. A ball catch must be fixed if it becomes too loose to hold the door closed or too tight so that it is difficult to open.
Open and close the door several times to observe the problem. Look at the ball catch and the strike plate (the piece that interfaces with the ball catch); check to see if they are aligned vertically and horizontally. Press on the ball to see if it moves smoothly on its spring.
Tighten the hinges of the door. Especially if the ball catch doesn't line up with the strike plate vertically this may indicate that the door is not square. If tightening the hinges doesn't work, the hinges may need to be reset so the door hangs squarely.
Remove the ball catch and adjust the depth clearance and the spring tension. Unscrew the ball catch from the door. Behind it are threaded areas that you can turn to adjust the depth by moving the ball farther in (for a fit that is too tight) or farther out (for a fit that is too loose). There is also a screw-slotted area you can turn to adjust the tension on the spring. Loosen the spring for less tension, tighten the spring for more tension.