can join friend and friend cant join..

Whenever my friend wants to join my minecraft world, he cant because it doesnt pop up in the "friends playing" list.. i have checked every single problems that could be causing this "íssue" including world settings, wifi, etc etc. i dont think it has issues between iOS and Android. i have also checked how to solve this issue and it still doesnt fix it. but it could be. please fix this or the game will be so boring not being able to play with friends.