Does not seeing the Prophet, peace and blessings be upon him, in a dream, despite wishing and hoping for that, indicate a lack of faith, or does it require greater effort, or is the reason for the lack of response due to past sins committed before becoming committed to religious observance?

Seeing the Prophet, peace and blessings be upon him, in a dream is a gift and a favor from Allah. Not seeing him is not an indication of a deficiency in a person's righteousness or faith. It is recommended to strive for greater uprightness in adhering to Allah's religion, following the Prophet's law, and frequently invoking blessings upon him, in the hope of seeing him in a dream and joining him in Paradise.