There is a sensor on the windshield side of the mirror. Turn the mirror off, cover that sensor and turn the mirror on. Keep the sensor covered and the mirror should dim. It should dim automatically from then on. I believe the calibration instructions are in the directions. I have three of these. Two of them I installed myself. I had the same problem with one of them and I did what I just described and it's been working ever since.
Good luck.
Good luck.