Couple possible troubleshooting steps to take:
1. Remove the head and battery.
2. Completely unscrew the tailcap (Do this over a table so no parts get lost)
3. Reseat the command tube (black tube inside the battery tube)
4. Verify the nub between the switch and rubber is sitting centered in the hole (check THIS post for visual confirmation and more instructions)
5. After you reassemble the tail switch and have the command tube reseated, tighten the tailcap all the way down.
6. Check the head driver retaining ring, loosen and tighten.
7. Test light.