A notification that is successfully delivered to the browser push service, may not reach the end user for various reasons:
deliverability
Web push subscription age affects delivery rates
This article investigates one of the reasons that negatively impact delivery rates for web push. First it shows that web push subscriptions can last many years without expiring, then it proves a negative correlation between the age of a web push subscription and its expected delivery rate.
Targeting only recently active users can increase delivery rate of web push notifications
There are various reasons that can prevent a web push notification from being delivered. One of them is the presence of subscriptions that are no longer in use:
Pushpad: improvements to deliverability and monitoring
Today we have deployed a new version of Pushpad which improves deliverability of push notifications and make it easier to find and debug any delivery failure.