I've only had my '15 Trailhawk a week but a couple of items for you:
1) if you don't press firmly on the brake, it might not engage.
2) if your climate control is working to moderate the cabin temperature, it might not engage or list "charging" even if you are at 14.4.
3) if you are using all the electronics (radio blasting while actively navigating, lights on), it might not engage.
Here in TX, it doesn't kick in on repeat lights when the AC is trying to correct for 90F outside
. Otherwise, it will work.
However, if you have the AC and radio off, lights off, and drive for a bit without braking, it should charge up inside 20 minutes of driving and start to work.
I had to figure out 1-3 with trial and error.