To confirm, what is the make/model of sensor? It appears to be the IQ Motion S-Line IQ1231-840, correct?
The motion LED does not light up when triggered unless it is in WALK TEST MODE
From the sensor manual, Walk test mode is activated by opening the motion detector back plate enough to release the tamper switch, then closing it again. During walk test mode the red LED on the front of the detector will light up every time motion is detected.
Use this feature to test the coverage pattern of the protected area. The motion detector will remain in walk test mode until there is a period of one minute of inactivity, meaning that no motion is detected for one minute, at which time it will return to normal mode.
If the motion does not trigger when you walk into its field of view, try deleting the motion from programming, then re-add it and test, any change?
How many feet away from the panel is the sensor and what type of material is the senor mounted on?
Programming can be found below: