Active Directory Question: Is there a reason why one user group would be accessible through LDAP and not another?

I'm getting member lists through an LDAP library in PHP, and there's one group that is returning 0 results. I can view it in Outlook and see that there are members in the group.

I have gathered member lists from other groups with no problem. I'm not an AD expert so I'm hoping someone has run into this problem before.

Is there a setting somewhere that makes it unreadable to something like this?

Thanks for your time.