Tip of the tongue phenomenon (TOT) is a memory deficit where a person is certain they know the piece of information, answer, name etc. but just can't recall it at the moment. There are two main explanations for it: direct-access view and inferential view.
The former can happen when the memory of the information isn't strong enough itself, but there is still a memory signal that you have come across the information at some point. It just hasn't been actually committed to your memory.
Inferential view on the other hand occurs when there are other memories blocking the retrieval of the information. You actually do know it. There is just too much related information that just happens to pop up preventing it to be accessed. It's like getting so many irrelevant results from Google search that the correct answer is buried way down the pages.
I like the assurance of the later type of occurrences happening more often by the age. It's just a proof that there is too much information accumulated in my head, that sometimes it's hard to fetch the correct facts at the time among all the other related facts. We just can't keep up indexing and processing all that information.