Depending on the methods used by the RAG you mentioned, if it uses an embedding model to calculate similarity with existing text and ranks them by score, it’s possible that the user’s query does not contain the word “banana,” or that the embedding model may have failed to associate the word “banana” with the user’s query.
While embedding models can capture meaning and calculate similarity to some extent even with words not present in the original text, they are not perfect.
One approach to address this could be to try using different embedding models to see the differences.