Tip of the tongue (or TOT) is the phenomenon of failing to retrieve a word from memory, combined with partial recall and the feeling that retrieval is imminent. The phenomenon's name comes from the saying, "It's on the tip of my tongue." The tip of the tongue phenomenon reveals that lexical access occurs in stages.
People experiencing the tip-of-the-tongue phenomenon can often recall one or more features of the target word, such as the first letter, its syllabic stress, and words similar in sound and/or meaning. Individuals report a feeling of being seized by the state, feeling something like mild anguish while searching for the word, and a sense of relief when the word is found."

See: Wikipedia

Since those who experience it feel sure that they know the answer - often described as 'a feeling of knowing' - it would seem to be a problem with accessing already-stored memories. The implication being that the fact that a memory has been 'stored' is itself also stored as a memory (perhaps analogous to an 'index card' in a library).

The tip-of-the-tongue experience (TOT) has intrigued psychologists for nearly a century. R. Brown and D. McNeill (1966) provided the first systematic exploration of the phenomenon, and the findings since their seminal study suggest that TOTs (1) are a nearly universal experience, (2) occur about once a week, (3) increase with age, (4) are frequently elicited by proper names, (5) often enable access to the target word's first letter, (6) are often accompanied by words related to the target, and (7) are resolved during the experience about half of the time."

Source : A review of the tip-of-the-tongue experience^{[ paywalled ]} Psychological Bulletin, 109(2), 204–223.

Despite extensive research going back at least as far as 1890 (Brown W. Principles of psychology. New York: Holt.) the mechanisms of the phenomenon remain unknown - though there are several theories, including :

• Direct-access view
• Blocking hypothesis
• Incomplete-activation hypothesis
• Transmission-deficit model
• Cue-familiarity theory

(see Wikipedia link above)

Note that the TOT phenomenon is not restricted to ordinary word retrieval - it can also happen, for example when trying to remember names, place locations, or even musical phrases (ref.)

Also see: Memoryplugin-autotooltip__plain plugin-autotooltip_bigMemory

inexplicable

"Although it is commonly accepted that learning and memory occur via enduring changes in neuronal properties such as synaptic strength within a network of neurons, many details of these processes remain unknown, including the mechanisms responsible for the persistence and maintenance of memory over long periods of time.