Any chance that commenting is switched off for certain member groups? You or other board admins can change the permissions in the board settings: https://help.trello.com/article/791-changing-permissions-on-a-board
If that does not help, feel free to reach out to trello.com/contact with a link to the board and the username of a user that is unable to comment, so we can check.
If you had to thrive a new habit during a lockdown, what would it be? Trello
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