Brian has had an issue with an alarm system for two years that drives him nuts. They are supposed to get push notifications in the app, but often they don't. Or one person gets them, and one doesn't. Leo says that iOS can be very aggressive at shutting down background apps. Notifications can also be dialed up and down with sensitivity to not get a notification every 10 seconds when the wind blows. So it could be dialed down too far. If you haven't visited the app in a while, it may just be shut off. Or the company's servers may be down. There's a lot of points of failure there.
But it also may be time to get a new alarm motion sensor system. Here are a few from the chatroom -