Why do GPS voice directions work outside the car but not while driving?

Sorry for the weird title, but the problem is weird. I bought a GPS app - Sygic - a few years ago and it has worked without a hitch through various iPhones and iOS updates, but then it didn’t a year ago. Well, not as long as I was sat in the car, driving. When I parked someplace and left the car, bringing the iPhone with me, it woke up again and started to speak. Back to the car, drove off and not a word.

It’s not only that app, though. The same goes for Google Maps. I haven’t tried Apple Maps since it’s a bit lacking in Europe.

Anyway, we drove up from Portugal to Norway in November and not a word from the app. Then we started driving back two weeks ago and lo and behold! The Voice was with me all the way to southern Sweden. The next day likewise through Denmark and into Germany. The third day it was the sound of silence again - except from when I step out of the car bringing the mobile with me.

I’ve read a lot of the common replies to the same kind of issues around here and elsewhere, but none of them make any sense, really, because they can’t explain why it works and then it doesn’t. No settings have been changed. So I’m completely mystified.

[Re-Titled by Moderator]