Excel 2007 : Using excel as check register and it will no longer sum

I use excel as a check register and have for years, with no problem until today.
Columns are as follows

action date Payee description debit credit balance
Wd 4/10/12 Abc Dry Cleaner laundry 25.00 100
dp 4/10/12 Bob check loan 100 200



Today I posted about 5 checks and then copied the formula down the balance column to the last entry. The sum only went to the line above the last entry, no matter what I did. Even odder is that as soon as I typed the date on the next line the balance filled in in the column above. I am not very good on the computer and I have no clue what the problem could be or how to fix it I am going out of my mind. I checked the formula from line to line and it looks good and I don't see anything other than what is usually there. Please help. Thanks.

could you attach a dummy version of your workbook? one would need to look at the formulas you are using to give you an accurate answer.