samba - Sep 2010 - samba ACL problems in some of the Wokgroup PC's

Dear friends,
  I am facing the problem while adding the  ACL user into subfolder security
permissions from some of the WORKGROUPS PCs . While adding  the ACL user itself
it is asking for samba login  credentials ( I did not notice this behavior in
other PC's ) and after entering it, it is displaying  user object not found.
Though "user2" user exists in backend. it is giving this error.

This is not giving any problem in some of the workgroup PC's and PC's
which are some in domain.

I have enabled samba log level 10 , while adding acl user task is going on , I
did not find any comparable errors between workable PC's and non workable
PC's.

Can you please suggest why only some of Workgroup machines are giving this
problem?.I found this is not specific to any OS . one pc which is having windows
XP another having windows 7 exhibiting this issue. workgroup is common
WORKGROUP.

Please suggest me . I am anticipating your reply.

Thanks
Suresh