Bookmarked Server with Default Channel - Permissions?

I have recently joined the admin team of a community and they have managed to disable the ability for normal users (or anyone who is not a full admin) to connect to their specified default channel in their bookmark.

Which permissions would effect this?

Or is it even the permissions? Could it be something else?

I'm a bit lost, so any help would be massively appreciated.

Thanks in advance :)

Bookmarked Server with Default Channel - Permissions?