For Subform on user input it does not seem to update main form unless you click another field first

For Subform on user input it does not seem to update main form unless you click another field first

Hi

I am updating a main form field from my subform

Main form = balance
each row in the subform is for payments

For testing only one row in my subform

If i enter a payment and click another field the balance changes and upon clicking update it is stored correctly in the main form

If i enter a payment (DO NOT CLICK ANOTHER FIELD) I can see the Balance change (the on user input script is working) but when i click update the value in the main form is not updated

Thanks