I don't see why you would use the same calculation for validation and as an auto-enter calculation, but it's not the reason the field isn't updating correctly. This is a basic limitation of auto-enter calculations--they do not update automatically if the field containing the updated data comes from a different table as is the case here.
Make this a field of type calculation and it will update correctly. You cannot set a validation rule on a calculation field so you'll need to explain what this validation rule is intended to do before anyone can suggest a way to manage that requirement while using a calculation field to display the sum of these totals.
I didn't really understand the difference between the auto-enter calculation and the validation calculation and would love to understand that better. I assumed that auto-enter is for when the record is created and validation is for each time the record is displayed.
I changed the field to calculation and you're right. It works perfectly. That was very simple. Don't know why I chose auto-enter calculation instead of a calculation field.
Thanks for the help.
A validation calculation is used to trap for data entry errors. It should evaluate as True or a nonzero number if the data entered is valid and as False, 0 or null if it is not valid.
So if it's a number field and the number entered can't be larger than 100, you might use a validation calculation such as:
Self < 100
so that an error message (and you can create the message displayed), will appear if the person enters a value such as 101 along with an option to revert the value if so desired.