Why is something relating to contacts not working?

What I know

Skype's contact system is very buggy. It can't sync between devices and at times contact requests won't even go through.

What I think is going on

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.

What this means

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