This is a phenomenon that occurs mostly among non-native English speakers.
The other day, 4 non-native speakers were discussing the USSR flag. All of them described it as red-colored flag with a hammer and a cutting knife (example: That hook-like knife used for reaping or simply a knife). Now this kind of description would surely bring up the image of the actual USSR flag but would not be treated as a qualified description.
When the topic digressed, I purposely mentioned the sickle in the ongoing conversation and everyone knew what a sickle was and all of them then said the reaping knife that they had described earlier was a sickle.
Now the question is: They could not associate a sickle (which they knew already) with the image of a sickle on the USSR flag while describing the flag but once the sickle itself was the topic they knew that the knife on the flag was in fact a sickle.
What does this communication-association-memory problem or issue called that occur mostly among Non-native English speaker. I don't know if it occurs among native English speaker at all.
Addition after the question being flagged as duplicate + already answered: They wouldn't necessarily say that it was on the tip of their tongue as "tip of one's tongue" suggest that they has inkling of the word but they just didn't come up with it during the original flag conversation. I want to make distinction
1.) During the flag conversation, they would not bring up sickle in any case as there is not association, although they know sickle in isolation. But they just can't associate it with the sickle that they know on the flag during the flag conversation. 2.) The other is tip of the tongue which i understand is they know that the image on the is sickle but could not utter as they were not able to recall the word. They just don't experience any recall phenomenon.
Would you still call it Tip-of-the-tongue (TOT) phenomenon (since the very phrase suggest that the association is made but it lacks utterance due to some sort of mental block (probably temporary) at the moment.