It happens.
It is why structuring your school year in a series of sprints makes the long distance we have to go do-able. We build in time, not just once a year, but intermittently, like rest stops on a journey so that we can look behind and ahead and make those corrections to our course that might be needed.
Charlotte Mason was familiar with this. She even wrote a letter to her 'Dear Bairns', former students of her teaching college, offering them wise advice.
"The knowledge that has been acquired with delight is not forthcoming when it is wanted, and the teacher loses heart and thinks that her term's work has been a failure."
Don't lose heart.
First, Mason offers a reminder to keep our priorities (and, I might add, our pride?) in check.
"Love of knowledge is a better thing than the small share of it which we can acquire, and it is better that you should rouse in your children a healthy appetite for science, literature, nature, art, than that you should furnish them ever so well with facts to be produced at examinations."
"There is a right as well as a wrong way of securing our mental property: it is there we know, any picture or idea received with avidity is a possession for ever, it cannot be lost; but the difficulty is that the owner of the piece of mental property is unable to recover it at will; some chance circumstance may bring it to light at an unexpected moment after having lain buried for years, but that is not by any means so useful as if the said piece of knowledge could be produced when it is wanted. A bucket in a well is very useful if you can draw up, but if there is no chain, why, you are just as badly off, so far as getting water goes, as if there were no bucket at all."
We have been faithfully reading good, living books and narrating. What might we have overlooked?
Mason's answer is rather simple. "It is the links of the chain that the eager and enthusiastic teacher very often forgets to join. When the next lesson on the same subject arrives she takes the children's knowledge of the last steps for granted and goes on from where she left off."
Mason compares it to building on 'shaky foundations.'
"All this may be avoided by the simple process of riveting each link by recapitulation: never be tempted to go on so long with the new work of a given lesson that there is no time to go over it; never begin a new lesson without ascertaining that the last has been thoroughly mastered step by step."
Be sure to secure the recall of what is known by linking yesterday's math lesson with today's or last week's episode with the chapter set for today's reading. Begin each lesson time by bringing minds back to the previous reading or lesson. "Tell me where we left off yesterday." This is not a lengthy step but a minute or so to take up the chain of the previous lesson. With the chain firmly in hand, you are then ready to take on new material, fastening it to the previous links and riveting it securely in place with narration so that "each last lesson is linked to the one before it."
Seen in this light, exams are not products we are asking students to produce but a third opportunity to strengthen the rivets in such a way that recall is made easy and knowledge is integrated. They narrate immediately after the reading, they recapitulate at the beginning of the next lesson after the elapse of a few days or a week, and they tell back at the term's end in their exams.
"If we absolutely and always and from the first secure the last lesson, I think we may be tolerably at ease about the whole series, as each last lesson is linked to the one before it and brings it to the surface in answer to a mental pull more or less vigorous."
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Notes: Bolded emphases mine. Mason's letter can be read in full at the Charlotte Mason Digital Collection at Redeemer University College.