Notifications Not Working for a Specific Forum Category

Product Support
3

Well, first try to figure out how many users actually have that particular category in the “watching” list. If you have the data explorer plugin installed you can run this query

-- [params]
-- int :category_id

SELECT
    cu.user_id,
    CASE
        WHEN u.admin THEN 'Admin'
        WHEN u.moderator THEN 'Moderator'
        ELSE 'Regular User'
    END AS user_type,
    CASE
        WHEN cu.notification_level = 3 THEN 'Watching'
        WHEN cu.notification_level = 2 THEN 'Tracking'
        WHEN cu.notification_level = 4 THEN 'Watching First Post'
        WHEN cu.notification_level = 0 THEN 'Muted'
        ELSE 'Regular'
    END AS notification_status,
    STRING_AGG(g.name, ', ') AS custom_group_names
FROM
    category_users cu
JOIN
    users u ON cu.user_id = u.id
LEFT JOIN
    group_users gu ON gu.user_id = u.id
LEFT JOIN
    groups g ON gu.group_id = g.id AND g.automatic = false
WHERE
    cu.category_id = :category_id
GROUP BY
    cu.user_id, u.admin, u.moderator, cu.notification_level
ORDER BY
    cu.notification_level

There can be several factors that affect notifications, starting from site settings, category security settings, rate limits, and so on.
The best thing you can do is to try to investigate real-world examples of users who are not getting the notifications they should. Maybe they have reached their daily quota of emails sent? Have you investigated the logs of sent , skipped, bounced emails?