First you should check if you have enabled the appropriate notifications in units menu – Settings – Notifications, also please check if you have the appropriate notifications in the Notifications tab. Do you receive other kinds of notifications in the application?
You can try to Log out and then Log in. You can also try to uninstall the application (your account will not be deleted) and reinstall it. When installing the application, you must give permission to receive notifications. Perhaps the reason is that earlier during registration you got into the update process.