This is due to bugs in the lobby server code in the server, the client, and the lobby itself. When I have some more free time, I plan on working with MM and EnEsCe to completely re-write all of the code and forever solve all of these problems.
Until then, there's not much you can do except use /lobby to try and get it in the lobby. If that works, I'd suggest writing or finding a script you can use (possibly in ARSSE) to have it execute /lobby in the server ever X minutes
edit: I've stickied this so that people can check it when they have the same problem, rather than constantly telling everyone about it.