Server randomly not allowing people to connect, hamachi server.

When I'm shutting down my server after playing on it for a while, when I restart it, it won't allow me or my friends to connect even though I haven't changed a thing. Suddenly after a couple of minutes or half an hour it's fine and we can join again. No mods or anything on the server, just a plain Vanilla Hamachi server. Only on 1.4.2 so far.