Make sure you are not recording or previewing a scene. Inputs are disabled during recording and for a short time afterward; the REC LED may flash yellow.
Check whether Post Delay is active for that scene. While active, triggers are ignored and the REC LED flashes yellow.
Verify the input wiring and that the sensor is actually changing state. See Input Wiring.
In configuration, confirm the input is enabled for that scene and not inverted unexpectedly.
If Pressure Lockout is enabled, the device may be waiting for adequate air pressure before it will run.