On some occasions the activation email is not sent
There's no reason for it to selectively send to users. Have your users check their spam folders or contact your host and have them check mail server logs to see if the email was attempted to be sent by your mail server.
when it is sent it doesn't work; it says you do not have access.
You're missing the public and published Joomla menu item to CBs profile page. This link is mandatory. Please see your documentation, which you can download from the Advanced downloads section, for configuration instructions.
krileon
