If you edit once an email content JotForm stops overwriting it.
If you would like it to reset it with the field changes made on the form, delete the email and it will create a new one automatically (we will remove that automatic creation, I am just telling you so that you don't get confused when you see a new one created after deleting the old one). The new email will have all of the fields and it will update itself with the changes made on the form. Until you edit the contents of the email. Once you edit the contents of the email, it will not be updated with the changes you make on the form.
I hope my explanation was not too confusing. :)