Where in the car is the USB plugged in for the power supply?
I personally use a 12 V to 5 V converter that fits into the 12 V socket and can supply a maximum current of 2.5 A. That is 12.5 W maximum power.
That is 12.5 W maximum power. As an iPhone needs between 5 - 15 W to charge.
Many USB ports in older cars do not provide this power and therefore this could cause the problem if a car radio USB port is used.