Multiplayer 'Unable to connect to world.'

My friends can't join my Minecraft world but I can join theirs. It would say 'Unable to connect to world'. I find it frustrating to go on there world but I would like them to join my world. When II transfer they still cant join my world and the same when I create a new one. Please fix this. It might be my Minecraft but if it is tell me how to fix it.