Borderlands 2 PC Steam Can't Connect To Friend

Me and my friend can’t play in a game with each-other. I’ve invited him to my game when it’s online friends only, when it’s public, when we’re using Hamachi with our firewalls disabled, when we’re not using Hamachi with our firewalls disabled/enabled, but NOTHING. Literally everyone else can join, minutes ago I had my lobby as public and TWO randos joined me, with 0 issue. Every single time it says “Could not connect to host”. I really just don’t understand how this game could not work for one specific person, but every single other person who joins me does with 0 issue. This game is really pissing me off.