I thought I remembered learning somewhere that in some cases (not all cases);
Forgetting Torah might be a consequence of not studying and learning when someone should have been studying and chose not to.
Something like; “If you choose to not learn for on day it will be forgotton two days” meaning that a person would have to work two days, to relearn what he forgot over one day, if he was deliberately negligent in not learning.
Work and other obligations are of course not negligence.
But if someone knew that a certain time, was the time he should have been studying, and deliberately decided it wasn’t important to “bother with”, that would be negligence.
Of course there are many reasons why someone might forget, and each person knows themselves and whether they are learning as much as they should.