In most cases this is caused by battery saving options on the phone. It can help to open the app, thereby activating the system again. Sometimes the phone automatically disables apps with infrequent use.
Did you install PsyMate on Android version 5 or higher? It could be that the phone automatically disables push notifications to save battery (the ‘doze’ function). Check if PsyMate notifications are allowed through the phone settings (Apps or Battery) and if the option ‘optimize’ is disabled. The location for these settings varies between devices. It can help to search online for doze function solutions specific to your phone type.
For phones with Android 9 or higher there is another setting you need to set up correctly. This setting is called ‘Adaptive battery’ and can be found in the settings under the item ‘Battery’
In iOS14 Apple added a new screen when you press and hold the notification too long. In this screen you can mute or turn off notifications permanently when using the 2 visible buttons. To keep PsyMate™️ working correctly you need to press ‘Cancel’ at the bottom otherwise the beeps can stop.