Are you having difficulty doing pull-ups and chin-ups? It could be your scapula and your back, but it could also be your arm muscles. These two muscle groups are responsible for the pull-up movement and you would have to train them both in order to perform pull-ups or chin-ups. Watch the video below to see how you can train them for strength and flexibility.
Whether or not you prefer the overhand or underhand grip method, both are great bodyweight exercises to target your back. Good luck and have fun training!