Hi Sherlin,
You can check the access rules and make sure that the user has at least “Read” access on that association.
Otherwise, it sounds like an issue in the logic, you can follow the logic flow to make sure that the “comments“ objects are created and associated with that particular ticket through the association that you use to retrieve, and if you are retrieving the “comments” in a different session/user you also need to make sure that the objects (ticket and comments) are committed to the database.