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? 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.