Temporary (Listen) Server help 2nd Question

Sometimes my friend will see my server in his serverlist and sometimes he won’t he isn’t able to join through favourites but he can join through my other friend on steam if he also either sees it on his serverlist or not. Its not my firewall Anti Virus or Router. If someone could give me a checklist of what could’ve gone wrong or a solution it would be nice. Please and ty :smiley: