Unable to Connect to Friends' Worlds

I am unable to join other friends' worlds, regardless of if they have a mod/resource pack in place on the world or not. It stays stuck on the "Locating server" screen until I click to cancel it. If there's a resource pack I have to download to join their world, no progress is made when trying to download it.I've had a friend join me easily once, so I feel that it isn't a connection kind of issue. Any advice?