Once you know how to identify the two bases of your style. Simply get in front of a mirror and position yourself so that your lower base and your upper base are positioned accordingly. This not only strengthens the leg but it also teaches the muscle memory the proper position. There should be a subconscious ability to get into this position because no matter what style you are using the two bases need to be aligned on top of one another. If you cannot obtain this position repeatedly it is impossible to repeat leverage and swing direction.
Flexibility is much more important than bowlers realize.
Much of the inability to repeat shots has to do with flexibility issues. There is no part of the body that should be left unattended to as far a flexibility is concerned but I will try and narrow down the areas I feel are the most important. Each of these areas should have complete freedom of motion. The knee follows the ankle, the hips follow the knee, and the shoulders follow the hips. They are all related, do not expect maximum performance if you cannot keep them in shape.