Account Locked Out

Why is the Account Locked Out tick box greyed out in a User’s Property even when Policies, Account, No account lockout radio button is ticked ?
Background: Windows NT Server SP5 PDC, 4 X Windows NT Server SP6a BDC’s.
Mixture of 400 NT Workstations, Win 95/98 & Windows 2000. KIX Login Scrips, Policies & Profiles from the PDC. 1800 Users