Apologies in advance as similar things have been posted regarding this but i have had no luck in resolving my issue. I have some forms (created using indesign) which are sent out, filled and returned. Upon return forms are completely blank (bar the signature field) however if the document is printed everything becomes visible.
This is usually caused when a non-Adobe app is used to fill in the form, such as a browser plugin or Apple Preview.
Instruct your users to only open the file in Adobe Reader for best results. In the meantime you can try exporting the form data of that file and then importing it back in to solve this issue with the files you already got.