The Subaru XV's e-Boxer hybrid system will only operate on electric power under speeds of approximately 25mph and offer support to the petrol engine above these speeds, but this will only happen if there is sufficient charge in the drive battery and additional demands on the powertrain do not restrict this function - operating the climate control in very cold or hot weather can cause the engine to fire. Driving style can affect how effective the hybrid system operates, with heavy braking and strong acceleration reducing the opportunity for regeneration to feed energy back into the battery. As for fuel consumption, the official WLTP consumption figure is not particularly strong at 35.7mpg, so to achieve close to this would require a very frugal driving style.

Ultimately if you believe there is a fault with the vehicle then having it inspected by the dealership or an independent Subaru specialist may clarify the situation for you, but hybrid vehicles require a particular driving style in order to maximise the benefit of both electrical and petrol power.