Most browsers (i.e. Safari, Firefox, and Opera) will only display the notification on the screen for a couple of seconds before going off screen.
This doesn't mean they have disappeared forever, don't worry! The have actually been stored onto your 'Notification Tray' (On OSX Operating systems only)
This is your notification tray, along with how your notifications will look when they are stored.
This icon is located at the top right of your desktop next to the date and time.
To find the Notification Tray, just click on that icon and your notification tray will slide onto the side of your screen (much like a website push notification).
Here you (and your subscribers) can see all of the notifications they might have missed when they were away!