P6 15.1 - Cannot change the data date when scheduling?

I know this has to be something I am missing, but I have scheduled a project once with a date of 23-JUL-2015. So I go in to schedule it again with a date after 23-JUL and it runs, but never seems to really schedule and then when you go back in to schedule it again, it isn't picking up the date you selected. It's still picking up 23-JUL. Thanks.