Every time someone sends you an instant message over Skype, the Skype application displays a notification that you’ve received the message. A small window pops up at the bottom of your screen.
There are a couple of reasons you might not immediately receive message notifications from your Skype contacts. First, if both you and the contact in question are on the P2P architecture, both of you must be online at the same time for your messages to display.
If Skype no longer informs you that you’ve received a new instant message even when you and your contacts are online at the same time (from one or more of your contacts), it means that the sender uses an outdated version of Skype with an expired security certificate. To solve the problem, ask them to update their Skype application to the latest version.
Important: For secure and the best possible Skype experience, always make sure you’re using the latest version of Skype.