Exported as pdf print or interactive pdf?
Make sure when select the button the type is set to text field in buttons and forms dialog.
But this is a text field and signature field. I am producing a 16 page document with either text fields to fill in or signature fields to insert a digital signature. When the PDF is produced (correctly by exporting to interactive PDF and then Saving as a Reader Extended PDF) the fields are becoming randomly signature fields when they were created as text entry fields. Something is clearly wrong but what is happening is extraordinary. I am using current up to date software and hardware.
I replicate your issue here on indesign cs6, then export to interactive pdf then from acrobat save as Reader Extended PDF>enable additional features, and there's no problem.
I do think after nearly a day of trying everything I may have found the reason and it is my error!
I have used ordinary boxes in my InDesign document instead of text boxes. InDesign allowed me to do this and did not tell me not to. This seems to have cured the problem at last, thanks for your input Salah.