Recently i found out that the notification system (or Stack Exchange Inbox) is not working properly. Basically in the last 36 hours no notifications appeared while some action, that should be notified, happened.
For example comments in my and other questions were not notified even if they were posted.
I was just guessing that there were some caching or server problems but today i posted an answer on SO. This answer got up-voted (i noticed that because while browsing other questions my rep count passed from 655 to 665). I also noticed that no notifications appeared. So (still in the page i was browsing) i refreshed the page 2 or 3 times and nothing changed. Then i got to the last question i answered an noticed it was really up-voted, but still no notifications.