There are three possible causes
User Error
1) The Octagon combat ring is not fully tightened.
Solution: Ask the buyer to follow the troubleshooting step line by line
Product Defect
2) The NL1835HP battery is defective
Solution: Ask the buyer to test the battery in another light that can use 18650 battery, or test the battery in a charger
2) The tail cap is defective
Solution: First verify that the light works with another 18650 and verify the battery is working in another light. After these two steps, send a replacement tail cap to the buyer.