This happens when the connection from your iPad or iPhone to your Mac is not complete. Typically this is caused by the USB cable or the Mac's USB port. If you are experiencing this problem try using another USB cable or a different USB port on your Mac. Duet strongly recommends using Apple Certified cables as these provide reliable connections and safe power supply.
In rare cases the Mac's internal USB hub may not be functioning correctly and you will need to see an Apple repair professional to diagnose.