Stop start systems will typically only operate when the vehicle conditions are correct, which commonly means that the engine is up to temperature, there is not a large electrical load on the vehicle and that there is sufficient charge in the battery. Given that your car is very low mileage for its age it is possible that it has been used frequently for short journeys which may have depleted the battery, but your recent longer journeys may have helped to reverse this. We would suggest having the battery health checked as a replacement may be required.