Sum field doesn't add up

Hi,

I made an expeses claim form in which people can fill out the expenses to be claimed. I has 4 subtotals... these totals automaticaly add up after entering the amounts.

There is also a grandtotal (this is the sum of the subtotals). Here is my issue.

The last amount filled in will be updated in the subtotal but not in the grand total. If I enter a new amount the grandtotal add up till the previous amount. The last entered amount is not in the grandtotal. :(

Please advise...

Yours

Jake