Copy/paste a Form from a PDF to a new one makes Calculation not working

Hi!
 
I have to make many (140 similar forms, each with 20 to 50 fields) PDFs, each one including a form. The forms are very similar, so basically, I'm copy-pasting from one document to the other, and making small adjustments to each as is necessary.
 
The form itself contains several fields that are summed. When I copy and paste all the fields from the original to the new document, however, the sum doesn't seem to give the correct result--some of the fields aren't added to the sum, even though they still appear in the calculation (sum).
 
This issue occurs on all copy-paste instances, but the results, the specific fields not being calculated seems random, so I have to go from field to field across many docs to make sure that the results are correct.
 
When I find that a specific field isn't working in the SUM calculation, I can delete it, copy-paste it back from the original, then it works as it should.
 
Can someone suggest how I can go about ensuring that my copy-paste process doesn't break the form?
  
Thanks!