Why is something relating to contacts not working?
Skype's contact system is very buggy. It can't sync between devices and at times contact requests won't even go through.
Skype used to be P2P-based. That means that contact data would have been stored locally on each computer, probably with some sort of handshake to signify a 'contact' relationship. I believe that contacts requests are still done with P2P to some extent, which means that there can be de-synchronization. Just to test, try adding someone on one device and removing them on another. Chances are the first device won't immediately register the contact being "removed".
If you know of a better explanation, please do correct me.
Any contact-related part of the API is not guaranteed to work. That means sending contact requests, accepting/denying contact requests, events relating to contacts, and others may not work instantly. You may experience an arbitrarily large delay when using any of them