The two most common reasons why a Maestro In-Wall sensor might not work immediately upon installation is either:
a) The device is wired to the neutral side of the lighting circuit (as opposed to the hot/live)
b) The device does not have a good ground connection

Note: If you determine that the switch is wired to the neutral side of the lighting circuit, it is not wired according to National Electric Code and may represent a safety issue by means of an electrical shock hazard at the lighting fixture. Lutron recommends that all devices be installed in accordance with all national and local electrical codes.

The ideal way to determine if either of these are the issue is to have a qualified electrician use an electrical tester (multi-meter) to verify the connections. However, the following are tests that you can run that MAY help you determine if you have either of these issues. Please be certain to turn the power OFF at the circuit breaker before removing, installing, or troubleshooting any electrical devices.

1) Check the connections:
a. Be sure that BOTH ground wires (a green wire, and a bare wire) are connected to the ground connection within the electrical box.
b. With power to the lighting circuit turned OFF at the breaker, trace the black wires of the Maestro In-Wall Sensor to the electrical connection within the electrical box. If you can trace either of those connections to a bundle of white electrical wires that are all connected together, then you are likely switching the neutral side of the circuit.
2) What happened when you first turned it on:
a. Did the product work one time, but now itís stuck? In other words, were the lights turned off at first, but you were able to turn them on, and now they are stuck on? This indicates that the device is switching the neutral.
b. Is the switch stuck in the off position? This may mean that the sensor does not have a good ground connection. Keep in mind, this may also mean that the unit was damaged upon installation Ė make sure that power is turned off at the breaker during installation and that you are only connecting it to the wires that were connected to the original switch.

Lutronís recommended solution in either case is to consult a qualified electrician to ensure that your installation is properly grounded and up to code. One other option is to instead use Lutronís dimmable in-wall sensor (model number: MSCL-OP153M) on a dimmable light source (e.g., incandescent/halogen, dimmable LED).