When someone has a "tip-of-the-tongue moment" problem, difficulty is occurring with this process of memory?

The answer is "Retrieval process of memory".
Tip of the tongue refers to the phenomenon of neglecting to retrieve a word from memory, joined with fractional review and the inclination that recovery is imminent. Its's name originates from the platitude, "It's on the tip of my tongue." The tip of the tongue marvel uncovers that lexical access happens in stages.