The software update can be run within a Standard users account it they have an administrators user name and password. I am thinking that if you set a schedule for an standard user and the account is logged on then software update will not attempt because that use don't have the privileges to change system information. The Software Update schedule is saved separately for each user of a Mac. Try leaving the administrators account log in and see if it works. Note: Thus another user's account may automatically download updates even when your account has them disabled.