Both the instances of the Doctor program start with the same context. The two cases differ by a word. Crestfallen and depressed are
synonyms. The chatbot recognizes the context of the sentence in the second case, by identifying the word "depressed". The chat bot is able
to identify the word "depressed" as the word and its context of use is present in the word-database of the chat-bot. Although "crestfallen" is similar
in meaning to the word "depressed", the chat bot is unable to recognize this key-word and thus asks a more general question. The chatbot is unable to figure out
the context in this case, and thus fails.
A possible suggestion is to carefully increase the word-database of the chat-bot.