Multiplayer keeps on saying "Unable to connect to world"

My custom worlds that I try to invite people to very rarely actually lets people join. We've tried turning down video settings, changing multiplayer settings, and different ways of the other person joining. We've never been able to find a definite fix. This issue happens when my friend tries to join on their Nintendo Switch and I am hosting on my Windows 10 desktop. We also both have normal connections to internet.