my infrared methane sensor is always decreasing ,can you help me?

i have bought some Pyroelectric Detector and ir bulb from a company, and made my infrared methane sensor.Now I had some problems

1 the data denotes the concentration of methane in our display is always decreasing ing in a short rang(about 700ppm). the principle for the detecting is ndir.who can help me, waiting your help urgently.