On some Android devices, there can be a problem with the Bell of Mindfulness feature which means that bells are not heard at the correct time.
Usually, this happens because the phone has battery optimization features that interfere with the Bell of Mindfulness.
If you experienced skipped bells, please try to disable the battery optimization features. You can find instructions to disable these battery optimizations for your specific device on dontkillmyapp.com.
If you experience any other problem with the Bell of Mindfulness, please see Troubleshooting the Bell of Mindfulness.