Creat fllable word form with editable mathmetical abilities

Question

I am trying to create an evrnt registration form. I can make the fillable form on word ok. What I cannot get to work is a table that allows the choosing of X # sites for X #of days at X $ for a totaling line at the end. I added the fx formula which looks loke =product (A1,B1,C1). It works once but if the numbers are changed, and no, I didn't check the box that disallows editing, the total doesn't reflect the new values in the cells. Please help. Tinker