system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't most humans remember everything they 'know'?

Why is it people can know something, yet not be able to recall that information at will? 

For example in an exam not remembering a specific equation, fact or date or whatever, only for it to come to you in the last 5 minutes - the knowledge must have been there the whole time, you just couldn't access it. 

Or learning words in another language, you'll "forget" them but instantly know the word again when you hear or revise it next time, but would not be able to give it when asked before that.
gpt
Memory generation and exactly how the brain works is still very much unknown. It is thought that the brain creates pathways for memories... The more pathways or references to an event or piece of information you make, the more likely you will be able to recall that information at a later date. That's why making rhymes up for complicated equations etc. Can help you recall them in exams. In regard to remembering later or when not under pressure or a piece of information/ memory seeming to appear from the blue... Perhaps the brain has come across a pathway previously made and triggered by by some kind of stimulus in the current environment.