Can you please tell me where the recipient is viewing the PDF? The signature is an annotation and not all PDF Viewers display annotations even though they are part of the ISO PDF standard. If the recipient is viewing in the iPhone/iPad email app or in Safari, then they won't be able to view the signature. Please encourage them to "Open In" Adobe Reader or another PDF Viewer that supports this feature!
If this does not help, please let us know. Thank you for the feedback!