My formulas keep resetting themselves? I've removed them from the list on clear forms

I made some totals, subtotals and a tax calculation field. However, when I enter the data it doesn't work properly. So I go back in, reselect the exact same fields I did the first time and then it works? However, if I add more numbers onto the form or manual erase one it doesn't calculate properly? The formulas only seem to work once?  I've removed all the subtotal, tax and total fields from the clear form button. Any ideas?