Fillable PDF will not save information inputed into form

I have a fillable form that is not saving what users input into the form.  The form is initally downloaded from a web browser, saved to the users desktop and then filled out.  After filling out the form, the user receives an error message saying the document cannot be saved.  I don't know if this is important but will include anyways, the document also contains a digital signature and a submit button. Any help on this issue would be great!

Thanks