This is obviously a known problem. You notice the extra instructions in the box for resetting the floodlight when it won't turn off. Kind of a pain, especially if light is mounted 25 feet up on side of house.
I'm about to get the ladder out and reset mine .. happens every 6 months or so. In case you're lost the "Important Notes" page ...
1. Set RANGE control to MAX.
2. Set DUSK delay to LIGHT.
3. Set TIME DELAY to 0.1.
4. Set THIS UNIT switch to SENSOR.
5. Turn off power at circuit breaker panel. Wait 10 seconds.
6. Turn power back on at circuit breaker (the floodlights will turn on again).
7. Wait 1 minute, then walk in front of Motion Detector.
8. Ensure that Motion Detector does not see any movement for 10 seconds, lights should turn off.
9. Set RANGE, DUSK, and TIME DELAY controls as desired.
(and don't fall off the ladder)