Missed push notifications

I created an automation in Smartthings app that sends a “notification to someone” on my Android smartphone when a door is opened (a zigbee sensor). In fact, when I open it I receive the notification on my smartphone, but not always!
If I go to see the timeline, I see that all the openings have been detected and not only, having also created a routine through IFTTT that is sent a notification with Telegram with the same sensor, these are all there.

What can the lack of smartthings notifications depend on?