PDF Security with Form Fields

Hi

I'm setting up secure PDF with Form Fields that allow people to answer questions in the PDF.

I've set to allow "Commenting, filling in Form Fields etc" and UNCHECKED the box to "Allow copying of Content"

My problem occurs when I open the secured document and add some text to one of the form fields, my MAC says "The original Document cant be changed, so a copy of the original has been created".
When I save that copy and re-open, the security setting seem to have been wiped and any of the Content in the PDF can be copied.What's going wrong here and how can I prevent this from happening?
Thanks
CB