In the Philips Hue app, go to Settings > Notifications to check if your notifications are enabled. If you're still not receiving notifications, open your phone settings to check if you have enabled notifications for the Philips Hue app. If it is disabled or notifications are being sent silently, enable them. You can test your notifications by purposely triggering one: walk by a motion sensor, open a door with a contact sensor, or wave in front of your camera, for example.