One of the major challenges when developing a new skill is retaining the skill. Using the skill to get better and retain it is the responsibility of the employee, but you as a manager can do your part to hold them accountable.
The Forgetting Curve is a graph that shows how quickly people forget new information. According to Ebbinghaus's Forgetting Curve, people forget about 80% of new information within 24 hours of learning it.
There are a few things you can do to help prevent forgetting:
Practice the skill regularly. The more you use a skill, the more likely you are to remember it.
Use the skill in different contexts. The more contexts you use a skill in, the more likely you are to remember it.
Space out your learning. Don't try to learn everything all at once. Instead, spread out your learning over time. This will help you to better remember the information.
Use mnemonic devices. Mnemonic devices are memory aids that can help you to remember information. For example, you could use the acronym "ROYGBIV" to remember the colors of the rainbow.
As a manager, communicate these methods and actually ask them about them. For example, don’t just ask how the training went, ask for the specifics of how they will practice the skill and how they will space out the learning.
By holding people accountable for these methods, you act not only as a better manager, but as a coach and trusted advisor as well.