Calculated Value Doesn't calculate Nemore!? Help?

Ok, first off i've been using FMP7 for about 2.5 months.

I'm making a 'cash register' the layout is based upon an invoice table and the items are viewed through a portal, to the items table. Several values such as subtotal, sales tax and total are on the invoice table and calculated on the fly as information is entered through the portal. When I opened up my file this morning, it no longer calculated the sales tax on the fly.. you have to click outside the portal. Subtotal calculates normally and the total calculates but is incorrect because it takes the values from sales tax and the subtotal. When I opened an older (1 day) backed up version of the file, it calculates on the fly perfectly. Sales Tax is a calculated value, and the formula as well as storage options etc, are the same as the old version. What did I do that prevented just the sales tax from calculating on the fly??? ANY information would be appreciative. Thank YOU