One is that it is too close to the gutter so the ball may easily drop in the gutter. The second reason is that your ball may have a slight curve and it may turn away from the pin. Instead, learn to hit these corner pins by sending the ball across the alley. If aiming for the left corner pin, for example, stand far to the right, and aim your ball toward the middle. This puts you lower to the floor and can help control the ball.
Many new bowlers make the mistake of turning their wrists, and their balls turn toward the gutter.
Instead, try to keep your wrist as straight as possible. If you have a spare, you will get 10 points plus the next ball. Therefore, the next ball after a spare is very important. Try to knock down as many as possible. Concentrate and put into place your best bowling skills on these attempts.