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.