Users cant change p/w in W2K domain

I administer a Windows 2000 network which was upgraded from NT4 domain by performing an in-place install of Win2K on the NT PDC. Since then we have had a problem with some (about 30) of our 300 or so users who are unable to change their passwords.When they attempt to change them, they get an error message stating that they are unauthorised to change their passwords. I have searched the Microsoft website, knowledgebase and Technet and so on without finding a solution to this. Any help that can get rid of this problem would be appreciated, I don’t want to have to keep changing thirty or so user password every month for the rest of my life! Plus I think that the network should work as advertised…