I can't get into my friend's map at MCPE

I try to enter the world of my friends in Mimecraft PE and the same error always appears, "Couldn't connect to the world". I've done everything that talks on the internet and more, and nothing solves this bug.

My friends manage to enter each other's world, but only I can't, and that happens to many people! HELP ME PLEASE!! (Cell phones are all Android)