Sheikh Uthaymeen رحمه الله
Why students lose knowledge they had memorised?

Question:
“Many students suffer from forgetfulness of knowledge which they had memorized and loss of benefits they read about. So what therefore is the ideal solution for this problem in your opinion oh virtuous Shaykh?”

Answer:
“The reasons for weak memorization are many. From them; sins. For indeed sins bring about forgetfulness. Allah تبارك وتعالى says:
{So because of their breach of their covenant, We cursed them, and made their hearts grow hard. They change the words from their (right) places and have abandoned (نسوا=abandoned, forgot) a good part of the Message that was sent to them.} [Surah Al-Maidah 05:13]

And Imam Ash-Shafi’y said: “I complained to Waki` about my bad memorization. So he advised me to abandon sins.
He said: “Know that knowledge is light. And Allah’s Light is not granted to a disobedient (sinner)!”
And from the reasons for forgetfulness: Is that a person does not read regularly what he has memorized, meaning he does not revise it. And the proof for that is the saying of the Prophet صلى الله عليه وسلم: “Read this Qur’aan regularly”, meaning revise it, “for, by the One in Whose hand is the soul of Muhammad, it is more likely to escape than a hobbled camel.” [Sahih Muslim]
Thirdly: Frequent preoccupation. When a person has many things that preoccupy him he’ll subsequently forget. Because the heart is a container and he is the vessel. And this vessel has a limit and a restricted capacity. If it is filled with something there is no space for anything else. Thus if a person has no spare time and is constantly preoccupied, those things which keep him busy a lot cause forgetfulness.
And also from the reasons for forgetfulness is not acting upon the knowledge. Hence if you act upon what you know then your action is considered implementation; implementation of knowledge. An example being: If you learnt that Wudhu has to be done in order and you perform it after that in order, then that is without a doubt implementation.”

Shaykh Muhammad ibn Salih Al-‘Uthaymeen, may Allah have mercy upon him
Source: Silsilah Liqa’at al baab il maftouh, Laqaa ul Baab il maftouh 197.

Shaykh saalih bin ‘Abdil-‘ Azeez Aali shaykh said: “There is no such thing as “Study time” for the student of knowledge! This is because all of his time is spent in seeking knowledge. In the morning and in the evening, his mind is preoccupied with the various issues of knowledge.”

~ Al-Manhajiyyah fee Talabil – ‘ilm”