Random systems can't save my form.

I have sent the same fillable form that I created in Acrobat X Pro to over 30 different users and all of them have the same computer system.  All have Acrobat Reader X.  The security settings have no restrictions.  Of the nearly 40 employees two of them were unable to save the form with changes.  What other factor could be causing this?

Thanks in advance,

irvy