Cannot log in on the NT domain server

I want to add a new user to the domain, I successfully added it but the client computer or prompts that the password is not correct. I check it many times to be exact 12 times and still same prompt. Old accounts are perfectly working, how is that so?
Is there a way an Administrator to lock the newly added user? (yes, new user account can be seen but password is blocked) if there is, I ask for the solution to UNLOCK it.
Note: Administrator left the scenario to me – i am now the new admin.