User not granted access if they are member of a group that is a member of an authorised group

I have an Active Directory setup where the intranet website should grant access to members of the group

OU=SiteAccess,OU=SiteGroups,OU=Groups,OU=Site,DC=x,DC=y,DC=z

and this contains both user accounts and other groups. I have placed this group into the "LDAP groups which allow automatic account creation:" section of the LDAP Groups settings.

Where a user is directly listed in the above group, they are correctly authorised access to the site but where they are a member of a group that is listed in the above group, they are not granted access.

From a long term maintenance point of view, it makes sense to have as few groups containing actual userrs as possible - for example, I have the IT department group inside the above group so that changes to the department members only need to be made in one place.

Does anyone know where I should start to look and improve the code to include this scenario?

Thanks

R