The answer to this depends upon your interpretation of the the motives for their responses. The response is self-effacing, implying that they feel they are, as you say, unworthy, or inadequate or merely humble, but in my opinion, the person doesn't want to hear feed back from you and is responding this way to shut down the conversation. Words for that are evasive or distant.
But why are they that way? They could be insecure - lacking the self-confidence that they could positively change in response to your request. You could categorize that under defensive. They could be disinterested - unwilling to invest the emotional energy to maintain a give-and-take relationship. You could call that apathetic. Or they could be fed up with the relationship - perhaps they feel you are unresponsive to their requests. You could call this exasperated, spent, frustrated, or emotionally depleted. There could be many other explanations. Good luck with that person.