The cable might not be properly connected to the car or the charger. Make sure that the cable is pushed all the way in, or replace the cable if faulted.
If the charger LED turns red after starting the charge, the RFID used might be unknown or inactive. The customer needs to make sure that no other RFID chips are present, and you can check if the RFID is active and make sure that "roaming" is enabled.
If the charger LED is red without the car connected, try rebooting the charger or ask the customer to turn off the power to the charger on their switchboard and turn it on again. The charger will reboot and become operational in 5-10 minutes, depending on the cellular reception in the area.