Is it because you are running the DB2 services under a local account (DB2ADMIN)? We found that DB2 doesn't mix local and LDAP authentication.
If the id that the services are running under is local (DB2ADMIN), then all authentication is local and LDAP doesn't work.
Set up a LDAP "service" id (this doesn't have to be an id you can log in with). Put this ID in the same "users" and "groups" that DB2ADMIN is in. Go into the Registry and make sure that the service ID has the same rights that DB2ADMIN has (create a token object, run as a service, ect). Go into the services and change the DB2 services from running under db2admin to the new service ID, start and stop, should work.