Practice will only make you master that which is practiced. You practice the wrong method and the more adept you will become at practicing the wrong method.
In short, mediocre salespeople are masters of the wrong method of selling. Mediocre teachers are masters of the wrong method of teaching. Mediocre speakers are masters of the wrong method of speaking. Mediocre managers are masters of the wrong method of managing.
Many organizations provide adequate training under competent instructors. But not everyone who receives instructions grows. The difference is in application. Most of us do not get to apply new learning. Without opportunity to apply, we go back to the old ways.
Most of us do not get the opportunity to learn from their mistakes. Take every action as a rehearsal to the next and you will gain mastery faster than you expect.