There are instances when even the most mentally healthy of us has trouble recalling a word during conversation, and it doesn’t only happen to older people, although it probably happens to older adults more frequently.
The question is “Why does it happen?” The only real answer is normal brain function. That leaves us with a myriad of possibilities, but there seem to be some that are more prevalent than others. First, multi-lingual people seem to have the problem more frequently than those of us who only speak one language. Your brain doesn’t process in words but in pictures, patterns and sounds. Words that have a similar sound, picture or pattern in the brain may conflict with each other creating recall problems. That strongly suggests that the more information that’s stored the longer it takes the brain to recapture and process it. Frequently, knowing the first letter or number of syllables in the word and repeating it mentally or aloud will solve the problem.
Second, brain plasticity stores information in order of priority, importance and need. Words that aren’t used often or that don’t fall into the area where faster than normal recall is essential might be stored off where it takes longer to bring them back.
Third, words that are used to express concepts require more processing time. These words can fall into any of three categories; words that are infrequently used, words that don’t have a picture, pattern or sound that brings them up quickly, and words that fall into both of these categories.
There is a fourth reason, but this takes us to a pattern where entire conversation threads are lost, the frequency of word loss is much greater, and words that do have pictures, patterns or sounds are lost. For example chair or chair-pair. If this is the situation you’re experiencing than it might be a good idea to consult a doctor.