samba - Feb 2007 - Active Directory Authentication working only for a subset of accounts

I'm trying to use samba with A/D integration for authenticating shares
(security=ads in smb.conf).  It works, mostly.  But there are several
dozen accounts that I have identified (and probably many more which I
haven't identified) for which it DOES NOT work.  I can see no
significant difference between the accounts, and I get mixed results
using wbinfo ( as reported below ).  Any advice?

smbclient -V
Version 3.0.23c-2


EXAMPLE WHERE EVERYTHING WORKS
=============================wbinfo -u | grep tester
  FOO+tester
wbinfo -n tester
  S-1-5-21-1708926621-995487588-1868020167-1151 User (1)
wbinfo -s S-1-5-21-1708926621-995487588-1868020167-1151
  FOO+tester 1
wbinfo -i FOO+tester
  FOO+tester:*:23827:20000:Kevin Test:/home/FOO/tester:/bin/bash
wbinfo -S S-1-5-21-1708926621-995487588-1868020167-1151
  23827

EXAMPLE WITH MIXED RESULTS
=========================wbinfo -u | grep testuser1
  FOO+testuser1
wbinfo -n testuser1
  S-1-5-21-1708926621-995487588-1868020167-1164 User (1)
wbinfo -s S-1-5-21-1708926621-995487588-1868020167-1164
  FOO+testuser1 1

wbinfo -i FOO+testuser1
  Could not get info for user FOO+testuser1
wbinfo -S S-1-5-21-1708926621-995487588-1868020167-1164
  Could not convert sid S-1-5-21-1708926621-995487588-1868020167-1164 to uid