I think I have resolved the issue. Arduino was sending the 20-second average data to a RPi. Here, the data was further averaged by 3 samples (=1 min in total) and then written to an airquality2.txt file. It was a floating point number with no restriction in the decimal places. When there were only 2 ( I think), WD picked the number OK and calculated the AQI. When there were more than 2 decimal places, it took 0. I changed my script to limit the decimal places to 2 and now it works OK.