User permissions

Hi everyone. I’m really battling to unscramble some user permission problems. I’m running Multisite so that users can setup their own blogs. Members with Author rights are able to create their own subsites, but they are unable to post/edit as they have insufficient permission to do so. They can, however, post on the main site. This needs to be the other way around! Users should be able to post on the subsites they have created, but *not* on the main site. Any ideas what could be causing the conflict? I’ve installed the User Role Editor plugin but this hasn’t helped to demystify the problem.