Hi tasseb,
the team edition’s push notification service is best effort only, so it’s hard to debug that and it could well be that the notification has been sent and not processed by and part in the chain. You can, however, monitor the notifications.log file (which resides next to the mattermost.log file) and you should at least see that a notification has been posted by Mattermost and if it has been acked by the push service.