LDAP group memberships not resolved correctly

My indico installation is unable to retrieve a complete list of group memberships for ldap groups.
Indico usually retrieves 1 to 3 group members albeit the groups contain more members. Does anyone have an idea what could cause this?

IDENTITY_PROVIDERS = {
'shib-sso': {
    'type': 'shibboleth',
    'title': 'LDAP',
   [...]
},
'ldap': {
    'type': 'ldap',
    'title': 'LDAP',
    'ldap': {
     'uri': 'ldap://ADSERVER:3268',
     'bind_dn': 'USER@DOMAIN',
     'bind_password': 'SECRET',
     'timeout': 30,
     'verify_cert': False,
     'page_size': 1000,

     'uid': 'sAMAccountName',
     'user_base': 'DC=uni-hamburg,DC=de',
     'user_filter': '(objectClass=user)',

     'gid': 'cn',
     'group_base': 'DC=uni-hamburg,DC=de',
     'group_filter': '(objectClass=group)',
     'member_of_attr': 'memberOf',
     'ad_group_style': True
    },
    'mapping': {
        'first_name': 'givenName',
        'last_name': 'sn',
        'email': 'mail',
        'affiliation': 'company',
        'phone': 'telephoneNumber'
    },
    'trusted_email': True,
    'default_group_provider': True,
    'synced_fields': {'first_name', 'last_name', 'phone', 'address', 'sAMAccountName'}
}
}