Verified answer

Ok so it counts who is trying to join. You have to be on the same router. If you both are on the same router, sometimes it's because a computer has a anti virus thing and if wont let you join, that happend to me. Or there is a problem with minecraft like a bug. Hope this helped

maximum common is that the port minecraft makes use of is blocked, set up the server on the different laptop, then verify the firewall settings on it and open port you have chose to incoming and outgoing, that would favor to do the trick