I have two tables each of which has a subtotal column and a total amount field under the table. One of the tables' subtotal column is the product of a price and quantity columns for each row. The total amount fields calculate the sum of the subtotal columns in the associated table. A third field exists that calculates the grand total of the two total fields.

When the price and quantities are entered or changed, the subtotals calculate correctly and the total field related to the table calculates correctly each time. However, the third field sometimes doesn't recognize that a total amount field has changed and, therefore, doesn't get updated with the new grand total. When I say sometimes, I mean that I can change the same field several times and the grand total field will sometimes update and sometimes not. It's not consistent at all.

Each total/grandtotal field is a number field that's readonly.

Here's an example: 

 In the same form a few minutes later, I changed one of the prices, and everything calculate correctly:

The GL Codes table isn't used in the calculation, so just ignore the value stored in it.

Any idea why this might be happening and how to fix it?

thanks!