I've been having this problem for a while now. My MacBook Pro (running Mojave) can see my iOS devices, but my iOS devices can't see the MacBook. This problem occurs intermittently (it was working fine hours ago). The last time this problem occurred, I was able to "force" the MacBook to show up on the iOS device by Airdropping something to it, but now that isn't working either. Is this a known problem/is there a solution?