My gmod server wont let more than one person at a time on it

I am having a problem with my 20 slot server. It wont let more than one person on at a time. So if im on than other people cant join and vice versa. Ive tried contacting the server provider about the issue, but they havent gotten back to me and im stumped on how to fix it.

what does it say when others attempt to join?

can you show a pic of anything?

Well when they join the server its just stuck at authenticating with steam and it just stays there for a while and sends them back to the main menu.

-snip- I missed the fact that it was rented

Is it a shared server? Is the physical hardware being used by multiple customers or does your server (even if it is virtual) have dedicated CPU / RAM / bandwidth allocated to it?

Other users might be causing the physical server to be unable to keep up with demand.