My sever never responds

Hey all!!!
I have tried to make a server in multiplayer and i make it and when i spawn it works fine but everytime i ask a friend of mine they tell me that the server says not responding and no1 can join cause server not responding whats going on my signal strenght i 40% 60% depending on what part of the house i am but it my internet runs very fast anyways is it the wifi or what please help