Cannot change user accounts

Hello,

I have a situation where as portal administrator I can change the details on some user accounts via the 'user administration' tab (eg. new password) but not others. It won't let me delete these accounts either and recreate them, but it will let me copy them, so it appears somehow these account records are readable but not updatable. Any idea's as to what is wrong and how I can fix it ?

Thanks,

Phil.