1. With the head of the D25C tightened all the way it will be in the mode where you can only cycle through Turbo and Strobe modes.
2. With the light on if you loosen the head slowly you will see the beam change.
3. Once the beam changes, then you are in the mode where you will be able to cycle through the rest of your brightness levels. (Low - Med - High - Strobe 1&2 - S.O.S. 1&2- Beacon - Low flash)
4. To cycle through the different brightness levels you half press the power button without clicking it.
Please try these steps and let me know what the outcome is.