AoA, the cng consumption of a car depends on a lot of factors, eg. engine size, condition, usage, the driver's foot, quality of the gas waghaira waghaira. your car, if its an old model, will continue to give you problems regarding the mileage. here r my suggestions,
get it tuned by sum good mechanic.
change the air filter and check the plugs etc.
get the mechanic to check it for leaks.
if it still doesnt give better mileage, phir aap ki bad-luck hi kharab hey!!! sell the car and get a duet!!!