Cannot join players / players can't join me

I've been trying to play with a friend of mine recently [I run Windows 10, they run iPad pocket edition], and whenever I try to join them I get a "Unable to connect to world" error. My friend gets the same error when trying to join me. The strange thing is that I was able to join them before, but now it's not allowing me. Is there a way to fix this?

SIDENOTE - This has being going on for about a month.