Our brains are like libraries, staffed by gnomes. You've got the gnomes who work in Intake/Processing, the Department of Recent Memory, the Department of Practical Knowledge (which consists mainly of things like language, reading, writing, walking, biking, driving, cooking, etc., is largely mechanized to save on gnome labor), and the Stacks. The Stacks are enormous. Everything you've ever experiences is in there.
There are two kinds of gnomes working in the Stacks. When you want something out of the Stacks, you have to send a gnome in to get it. Those are the Retrieval gnomes, and they work in teams. When they're not running to get the information you're looking for, they're waiting at the Stacks desk ready to dash off at the first hint of a request. They're generally overworked. Then there are the Maintenance gnomes. They're work back in the Stacks, keeping things in order, making cross-references when necessary, and when no one is looking, sneaking off for a quiet cup of tea in the farthest reaches of the Stacks.
Whether or not you get the information that you're requesting depends largely on where it is in the Stacks, and how busy the Retrieval gnomes are at that moment. It can take a while to find the right memory. Sometimes you get tired of waiting and re-prioritize your search privileges. That's what happens when you can't remember something. Sometimes, the gnomes make a decision and they keep one of them going on the request, even if it takes much longer than you were willing to wait. That's what happens when you wake up and know something you couldn't remember the day before.
Sometimes, there will be a maintenance gnome working right where the information happens to be, and when you request it, they pop their heads up over the stacks and shout it out to the front desk. Maintenance gnomes are not diplomatic, and they love to occasionally embarrass the Library's Founder (that's you). They are also a little bored with their jobs, so they sometimes go on unofficial searches. If you idly wonder something in passing without particularly caring about the answer and then you have the information pop into the front of your head, that's because a bored Maintenance gnome went looking for it.