iOS: Notifications and iBeacon - ios

Question

I have an app that needs to send notifications when user is out of range of iBeacon. The problem is that user becomes notifications randomly: sometimes it works, sometimes it doesn't work.
My code:
// this function I call in viewDidLoad for my UIViewController
func startMonitorig() {
let beaconRegion:CLBeaconRegion = {
let r = CLBeaconRegion(proximityUUID: UUID(uuidString: "XXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX")!, major: 1, minor: 6, identifier: "com.myapp.identifier")
return r
}()
locationManager.startMonitoring(for: beaconRegion)
locationManager.startRangingBeacons(in: beaconRegion)
}
override func viewDidLoad() {
super.viewDidLoad()
locationManager.delegate = self
locationManager.requestAlwaysAuthorization()
startMonitorig()
}
AppDelegate:
let locationManager = CLLocationManager()
var window: UIWindow?
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
// Request permission to send notifications
let center = UNUserNotificationCenter.current()
center.requestAuthorization(options:[.alert, .sound]) { (granted, error) in }
locationManager.delegate = self
return true
}
extension AppDelegate: CLLocationManagerDelegate {
func locationManager(_ manager: CLLocationManager, didExitRegion region: CLRegion) {
guard region is CLBeaconRegion else { return }
let content = UNMutableNotificationContent()
content.title = "Forget Me Not"
content.body = "Are you forgetting something?"
content.sound = .default()
let request = UNNotificationRequest(identifier: "ForgetMeNot", content: content, trigger: nil)
UNUserNotificationCenter.current().removeAllPendingNotificationRequests()
UNUserNotificationCenter.current().add(request, withCompletionHandler: nil)
}
}
I'm curious why it's like that and what I'm doing wrong? What can be the reason?