Actual cost progress on P6

All,

I have a broblem managing actual costs on P6. When I create a new schedule, and after having assigned to each activities an expense (or a non labor resource), i try to track the progress on the S curve. My problem is given by the fact that during the months I loose the evidence of the previous monthly updated done. I mean, if at the end of the month one I update the programme assigning the actual expense, on the second month, when I make the new progress, the value for the actual cost I fixed on the month before has changed. Consider that every month I create a new baseline. Can anybody help me? I think it depends from some setting on the programme but i haven't found any proper solution