Multi Site Question

Sorry ahead of time if this sounds like a silly noob question.

I've got multiple sites running from a single installation of Drupal. Each site has it's own domain name and own database. I'm sharing some tables because the three sites are closely related and belong to the same client and we want to be able to have a single login and allow users to access and edit profile information for any site.

The problem I'm having though. Is that login only seems to work from the master domain. If I try to login from one of the slave domains it's not working. However If I go to the master domain I can login and then access any of the three sites.

I'm guessing that there's probably a really simple solution to this problem. But is there a way to set this up so that I can login from any site?

Many thanks

riki

NB: acp_01 is the master.

$db_prefix = array(
'default' => 'acp_03.',
'access' => 'acp_01.',
'profile_fields' => 'acp_01.',
'profile_values' => 'acp_01.',
'users' => 'acp_01.',
'sessions' => 'acp_01.',
'role' => 'acp_01.',
'authmap' => 'acp_01.',
'sequences' => 'acp_01.',