Do you have the Mail Queue enabled? It is a Scheduled Task and it may delay notifications being sent.

Are you testing with multiple accounts? Accounts will not receive notifications for their own posts.

Are you using a host account? Host accounts are not regular users in DNN and might not receive notifications.

Have you configured the SMTP settings in the Host Settings menu? These settings are used to send outgoing emails. If your site can send emails such as password reminders and registration information then it is configured correctly.

Is your mail server configured correctly? The module does not have any known issues sending notifications. However, we have commonly encountered mail servers that interpreted the outgoing notifications as spam and blocked the request, or are otherwise not configured correctly.

Are you getting any errors in the event viewer? Please view as the Host user and look for any error messages regarding email.

Have you configured the security settings on the forum correctly? In the Control Panel > Forums > {Forum/Group Name} > Security tab make sure the icon is green in the Subscribe column next to the role containing your test users.

Are the test users subscribed to the forum or topic? Make sure there is a check in the box that says "You are subscribed to this forum. Click to unsubscribe." You can check a user's subscription status by clicking My Profile > Forum Tracking. There will be a green check next to forums they are subscribed to.