Can no longer edit form fields

I have a form that I use to send to customers. When they pay me, I add their payment to their next statement through a form field, and I make the field "read only" until I want to update it. The field is used in a calculation on the form itself (to subtract the amount they have already paid to create a new total). I've been doing it this way for years with no problem. 

Now when I take the form off "read-only," I can't edit the field at all. I've tried creating a new field and renaming it. This works for a time, but eventually, it stops working. 

There is a signature field on the form, but it's blank. I don't understand why it's flattening, if that's the case.