Thanks for your responses,
I changed the testing environment to another restroom with low ventilation, now sensor returns ammonia value.
But as the range of the Sensor is 0-100 ppm I am having the doubt that the mild range of Ammonia (0-2 ppm) was not measured with good Accuracy (Now Sensor gives 0-1.3 ppm in Testing Environment).
Can you guys suggest me some technique verify the value measured is correct! or is there any other sensor available to measure mild level of Ammonia presence.