Diagnosis
You're likely in a specific brushing mode that overrides the intensity button, or the brush is stuck in a default/error state.
How to Fix It
Cycle the mode button first. Press the power button to start brushing. Then press the middle (mode) button to cycle through Clean, White+, Gum Health, and Deep Clean+. Now try pressing the intensity (low/high) button. The change is subtle but you should feel a change in vibration pitch.
If That Didn't Work
Perform a system reset. Turn the brush off. Hold down the power button AND the mode button simultaneously for 10+ seconds until all lights flash. This reboots the system and often clears setting glitches.
Heads Up
In the 'Gum Health' mode, the intensity is automatically modulated and pressing the intensity button has no effect. This is by design.