This problem has dozens of reasons. In most cases plugin user (website admin) doesn't test it properly or doesn't know some important things:

1. Before reporting this issue on support forums please make sure your server email-sender works well. Make sure you're getting emails from your WordPress (on registration or on other cases...).

2. Some hosting services, or some Antispam plugins try to cache emails and those are not being sent immediately. You should wait 1-30 min (e.g.: on some GoDaddy hosting plans you receive emails in 45 min).

3. If you're using Gmail Fetch services, go to Gmail Settings > Accounts and Import and click on "Check mail now" link to load all new comments from the website email server.

4. The first email is the Subscription Confirmation Email, you should click on the confirmation link and confirm your subscription to certain post/comment. If you don't get the Subscription confirmation email and don't confirm it you'll not get future comments. Sometimes the confirmation email goes to spam-box and user don't check it. You should check in spam-box as well.

5. Test it as real as possible. You should use new IP, new emails and new accounts. The best way to ask your friend to post a comment with Subscription and Subscription Confirmation. Then reply to him.

6. Make sure you don't reply to your comment if you test your subscription notifications.

7. There are three subscription options:

-- Top Subscription Bar:
----- Notify of new follow-up comments
----- Notify of new replies to my comments
-- In comment form:
----- Notify of new replies to this comment

Make sure you test the correct subscription option. If you subscribed to one comment you'll not receive an email notification for other comments.

8. If you have Antispam or Subscription plugins please deactivate them before checking.

9. Please read the following sections in wpDiscuz documentation here:

• https://wpdiscuz.com/docs/wpdiscuz-7/plugin-settings/subscription-and-user-following/
• https://wpdiscuz.com/docs/wpdiscuz-7/wordpress-settings/comment-subscription/

10. Also this may be an installation issue. You should check WordPress database tables and make sure this table exists: