Why doesn’t PRACTICE work?

Why doesn’t Practice work?  Children can work for years on handwriting, yet the quality of their writing does not improve.  And for regulation, behavioral interventions, complex systems of rewards and punishments often do not change a child’s behavior or regulation. Also, why can’t we practice our way to professional level sports.  I believe this is due to our neurological sensory processing style.  We each have a unique way in which we take in, organize and use sensory information for skill.