If one of your pool lights is not turning on while the other is functioning properly, follow these step-by-step troubleshooting steps to identify and resolve the issue:

1. Check the Bulb: Start by verifying whether the bulb in the non-working light is burnt out. Sometimes, bulbs can fail without any apparent reasons. Remove the housing screws to access the light, and inspect or replace the bulb if necessary.

2. Inspect GFCI Outlet: Ensure that the Ground Fault Circuit Interrupter (GFCI) outlet for your pool lights has not tripped. This can often be the cause of pool lights not functioning. Reset the GFCI outlet if it indicates a trip - learn more - learn more.

3. Check Circuit Breaker: Next, look at the circuit breaker that supplies power to the pool lights. If the breaker has tripped, reset it by turning it off and then back on. Unplug or turn off any devices that might have caused the trip - learn more - learn more.

4. Testing Voltage: Use a multimeter to check for voltage at the junction box after the transformer. You should see between 12-14 volts. If you do not see this voltage, there may be an issue with the transformer or wiring - learn more.

5. Examine wiring connections: Check all wiring connections for corrosion or damage, especially at the light fixture and junction box. Ensure that wire nuts are secure and that there are no frayed wires.

6. Inspect Relay Panel: If applicable, disconnect the load side of the relay that controls the pool lights and try to reset the GFCI. If it resets successfully, then the issue could be within the relay panel - learn more.

7. Professional Evaluation: If you've gone through all the steps and the light still doesn’t turn on, consider contacting a professional pool technician to diagnose the issue more thoroughly.

By systematically checking these areas, you should be able to identify the cause of the malfunction and either resolve the issue yourself or gather enough information to seek professional help.