A common mistake of novice bowlers is refusing to adjust. You can throw six strikes in a row and be bowling the game of your life, but that same throw won't necessarily hit number seven or eight. In order to consistently score high, you need to be able to read the lane conditions, and then adapt to them. These tutorials will help you figure out when the lanes require a certain adjustment, and how to apply it.
When you're missing left or right, try a lateral adjustment to compensate.
Another way to combat missing left or right is to move forward or backward.
Use the oil to your advantage by changing the speed of your shots.
Use your index and pinky fingers to control the amount of axis rotation on your ball.
When all else fails, maybe it's time to try a new ball.