Two of my friends are in the same house and I am in a different state. When I host a server, they can both connect to me. However when one of them hosts a server, the other person can not connect but I can.
I hosted the server and both persons A & B connected.
Person A hosted a server and I connected but the person B could not.
Person B hosted a server and I connected but the person A could not.
Tried several times.