Check these in order
- Settings > Alarms > Ignore Do Not Disturb is on. This is required in addition to the setting on the alarm.
- The alarm itself has Ignore Do Not Disturb on (or its group does).
- Alarm Volume in Settings > Alarms isn't set too low.
- Notifications are allowed for the app (Settings > General Settings > Notifications for alarms).
- The phone isn't stopping the app in the background (see "How to stop your phone from closing Notification Tracker").
If it still doesn't ring, see "Why isn't my alarm ringing?".