Can't invite friends

So I made an island for my gf on Minecraft mobile, however, she plays on Xbox. My problem is that when I invite her to a game it doesn't work or she just can't join the server in general. I feel like this is a bug because it worked one time before and it never did again since. We've done everything right when it comes to inviting someone to a server. I may be doing something wrong, but I don't feel like I am. Thank you! Have a nice day