Which version of Skype are you using?
Which platform (Windows, Mac, Android...) and version of Skype is the other side using?
Please make sure that both you and the other side are using the latest Skype version.
You and the other side can try with the latest 6.7 version of Skype that you can download here
If the other side is using Mac, it needs to be at least 6.2 version.