Haven't played, but most of games have same problem...
If the player doesn't have same map you are hosting, he wont be able to connect
If you dont have newest updates installed or the guy that wants to connect has older update than yours he wont be able to connect again...
Also you AV, Firewall, or anything of that kind might be blocking it... When I host / connect I always turn it off all.
Also your modem might be causing it, or your upload speed is not satisfying the limit...
Also, If the game is pirated, that might be problem too
Post some info, maybe I can help somehow