Sorry for the inconvenience. We will definitely resolve the issue for you as quick as possible. Could you follow steps below which may resolve your issue?
Fenix PD35 tac has two operating modes. The front side button is disabled when it is in tactical mode.
1. One is called "Outdoor mode": you can use the tail cap to power on/off the light and tap front side button to change the brightness.
2. The other is called "Tactical mode": the tail cap can access low/high/strobe function through quick tapping. The front side button is disabled in this mode.
3. To switch between these two modes, turn on the light then press and hold down the front side button until the light flashes twice on low output. You can enable/disable the side switch through this method.
Please give it a try to see if it resolves the problem. (check the manual for details)