I purchased my Inspiron December 2016. I've had several issues with Hey Cortana not working at all or occasionally after a reboot.
The only way Hey Cortana would work however, is when I have audio or video playing in the background.
It doesn't matter if I had headphones, playback volume down or muted so it wasn't because of ambient noise.
Issue was reported issue to MS Insider 3 moths ago.
All drivers were up to date using Dell update for my device. I then did a clean installs of both Windows AU, re-ran Dell update as well as Creators Update and the newest audio drivers from Dell didn't help.
I finally went to the Realtek site and download their drivers directly to the latest revision 2.81 version Jan 2017 solve the issue. The drivers for the i5559 need to be updated.