If none of the above, very good suggestions work perhaps you may be wandering into seldom seen error territory.
Possibly corrupt update downloads if your wifi is spotty? You could try a different wifi or manually download the update on your PC and transfer it to the root directory of your Kindle via USB.
Perhaps your device didn't get the mandatory 2016 cert update, some random file got corrupted,
et cetera. You could look into logs, a
serial port adapter is one way to get a detailed look at what your Kindle system is doing and help pinpoint what is failing when your Kindle tries to update.