not getting readings from PM2.5 laser dust sensor

Hi, I have 4 units of the PM2.5 laser dust sensor (https://www.dfrobot.com/wiki/index.php/ ... KU:SEN0177).

I tried running the sample code on an Arduino Uno, initially I got this error: avrdude: stk500_recv(): programmer is not responding

Managed to get the code to compile and upload after I went to the Arduino IDE settings and checked the following boxes:
Show verbose output during: 1) compilation, and 2) upload.

But after that, the output in the serial monitor shows zero for all three readings (PM1.0, PM2.5, and PM10).

I also tried modifying the code to use SoftwareSerial, so that I could try connecting the dust sensor adapter's RX and TX pins to digital pins on the Arduino. However, I got the same output with that code, too.

Here's my modified code: https://www.dropbox.com/s/5s2ejvlkhulun ... r.ino?dl=0

I'm getting the same output from all four dust sensors, so it seems like this might not be a hardware issue. (I hope it isn't a hardware issue!)

Any ideas on how I can troubleshoot this issue?