This is typically an issue when a person uses a non-Acrobat product (Preview on the Mac or NitroPDF on Windows, for instance) to fill out the PDF form.
Somewhere at the beginning of your form, let the users know they must use Adobe's free Reader or one of their commercial products: Acrobat Standard (for education) or Acrobat Pro to complete the form. That, or just convert it to a web form, with instructions for them to print each page as they move along.
There could be a different reason, but this is the most common.
I hope that helps,