Opening the book…
Most failures to learn are failures of transmission. Your first explanation is built out of your own experience, and if it doesn't land, repeating it louder changes nothing. Trying a second and third route — different words, different medium, different direction — usually finds the one that fits how that person actually takes information in.
After a failed explanation, change the form, not the volume: describe it, then show it, then have them do it while you watch. Or reverse it — ask them to explain the task back to you and correct the version they produce. Keep track of which route worked, because it will work again with that person.
Some gaps are about willingness, not comprehension; if someone can explain the standard perfectly and still doesn't meet it, this is a different conversation. And when the stakes are safety or money, teach later and remove the risk now.