Before trying a PIR sensor, you might want to adjust the
Settings, Motion Detection
Frame Change Threshold to a lower number (Mine are usually down around 0.1%)
Do not use Maximum Change Threshold (leave at default 0)
Set Auto Threshold Tuning to "Off"
Turn on Auto Noise Detection and Despeckle Filter
Turn on Show Frame Changes to see what, if any motion, the system sees while testing the settings. Don't leave it on all the time, though.
If your minimum motion frames is too high, it won't see anything.
If the frame rate on the camera is too low, it won't see anything.