naming with service tag

When I create an image  with just windows  and use the  option to join a domain with the name M(servicetag)s I get the desired result of the computer joining the domain with m325jmj2s  as its name.  after I add all my software and image the same machine  I get the default name of M123xyzS or whatever the default is.   If I use random naming that works fine.   how can I find out what the problem is?