How does a coach handle difficulty in skill acquisition or skill patter degeneration? There are infinite answers to this question. It depends on the sport, on the athlete’s age (the older they are, chronologically and sports-wise, the longer the skill retention time), their natural skill acquisition ability, current and previous injuries and we could go on. My point is that there is no recipe.
Understanding the foundations of motor skill learning allows the coach to create a strategy tailored to that specific athlete.
The first example is Zach’s story: he is a relatively experienced powerlifter who competed in a federation that required the bench press to be executed with the head on the bench and no head movement, according to the rules. In his previous two competitions, he had two our of three bench presses disqualified because he raised his head. He couldn’t control it.
This is what we would qualify as skill degeneration or acquisition of new (undesirable) pattern.
DISCLAIMER: this is not an article. For evidence-based articles by others, see the list below. For evidence-based articles by me, click on my article link. This is just a tiny coaching log.
Moving on: that means we had a problem at the autonomous stage. He had acquired an automated and undesirable movement pattern.
My approach was to focus exclusively on the bech press for 3 weeks, with daily practice at high volume, very low intensity and increasing speed. This is the split. During the following weeks I will discuss the three week program composition.
Many things happen during motor skill acquisition. The conventional idea that it progresses in linear manner is not wrong: it’s just far more complicated and is now considered to be a permanent neural process.
Whatever the new consensus will be, the good, true coach must master the science and be confident enough to apply it creatively. No two situations are the same.
For evidence-based articles, click on the “articles” link on my author page: 
Some reading:
Understanding motor learning stages improves skill instruction
How does the acquisition of skill affect performance?
Stages of motor skill learning
