Notification not working

I have a Xam.Mac Cocoa application, created using Visual Studio.app. I have a ClickedButton method wired up to a button, and I'm able to do things like create modal alert dialogues, save a string to pasteboard, and manipulate some textfield.StringValue. For some reason, I can't get a notification to pop up. Not sure why this isn't working:

var notification = new NSUserNotification();

// Add text and sound to the notification
notification.Title = "25 Minutes is up!";
notification.InformativeText = "Add your task to your activity log";
notification.SoundName = NSUserNotification.NSUserNotificationDefaultSoundName;
notification.HasActionButton = true;
NSUserNotificationCenter.DefaultUserNotificationCenter.DeliverNotification(notification);

or

var myNotification = new NSUserNotification();

myNotification.Title = "Hello World";
myNotification.InformativeText = "This is an informative text";
myNotification.DeliveryDate = (NSDate)DateTime.Now;
myNotification.SoundName = NSUserNotification.NSUserNotificationDefaultSoundName;
NSUserNotificationCenter.DefaultUserNotificationCenter.DeliverNotification(myNotification);

By setting a breakpoint, I can see my code reached a line after this stuff, so it's not like my thread is hanging on some other call before getting here. Simply, nothing happens.

Except once. Mysteriously, one time, I got a notification to pop up, but have never been able to repeat that.

Visual Studio Community 7.2.2 (build 11)
Mono 5.4.1.6
Xamarin.Mac 3.8.1.0
macOS 10.13.1 High Sierra

Thanks for any suggestions...