When I've already launch a world,I can't join the world of my friend.

When I want to go to my friend's world while I have already launched my world, when I leave this one and try to join my friend's world, it does not work and a screen constantly tells me the same thing: the waiting time has been exceeded. this happens to me all the time and I have to restart my game every time and go directly in my friend's world,I've not problem with my internet connexion,please patch this glitch.