Diagnosis
The mode selection button or the control circuit might be stuck, or the brush is defaulting to its primary cleaning mode due to a glitch.
How to Fix It
First, let's rule out a simple button issue. Make sure the handle and your finger are completely dry. Press the mode button with a firm, distinct click—don't just tap it lightly. Cycle through all the modes (Clean, White, Massage, Sensitive) several times, listening for a change in pitch. The Sensitive mode should be noticeably gentler.
If That Didn't Work
Perform a factory reset. With the toothbrush turned off, press and hold both the power button AND the mode button simultaneously for about 10 seconds. You might feel a double vibration. This reboots the micro-controller. Try the modes again.
Heads Up
If it's still stuck on one intensity after a reset, the electronic switch is likely faulty. The motor itself only has one speed; the modes work by rapidly pulsing the power. If that circuit is damaged, you're looking at a replacement handle.