Users being 'dropped' from Essbase Security

Hi all

We're having a strange issue with Security in Essbase wherein users are being dropped.  We're going through the process of adding the users in Shared Services, with both Native and External LDAP (Lotus Notes - I know, don't get me started).  All groups are Native (we're not using External LDAP Group support).

Upon adding the user, and assigning the requisite group or application permissions, users are occasionally then dropped.  The issue becomes apparent when said user attempts to login and obtains #NoAccess from Essbase.  They're able to login to Planning and navigate webforms, which suggests the authentication lookup to External LDAP is functioning properly, as is Shared Services/Planning Security which allows them into the requisite forms (or, the Workspace at all).