Marking fields as read-only when signed - unable to untick

Hi folks,

I am using Adobe Acrobat Pro XI.

I have a form with Signature fields that have been set to make certain fields read-only once the signature field has been signed. I can bring up the Properties window with the list of fields to choose from and am able to "Select All" and "Unselect All" using the button but am unable to tick or untick individual fields. When I click on a field it i highlighted but I cannot "tick" it.

Any ideas why this would be????

Thanks.