I can't connect to many of my friends yet they can connect to me. What are the main reasons for people not being able to connect? Is it something to do with my Firewall? My Internet Firewall is low and I'm pretty sure my PC Firewall accepts the program.
It's strange that you can connect to them but they cannot connect to you when using Hamachi, since by definition you can reach each other via LAN.
I would like to help but this sounds like a case of Paradox being incapable of writing decent Netcode. Every paradox game has had bad Netcode and been unstable, had connection issues, etc. They claim to have spent a lot of focus on the EU4 netcode and its still not reliable.
Basically, I suggest you just connect to your friend, since the problem may well not be on your end. The mysteries behind paradox multiplayer don't tend to get solved.