If you do not feel a vibration after connecting the device, please try one of the following:
Make sure it's on Training Mode (Tracking Mode doesn't vibrate).
Sit/stand up straight and choose 'Set Posture' in the app to calibrate it to your posture (or double-click the power button on the device).
Choose a suitable trainer (the 'Stationary' one, for example).
Now, when your avatar slouches past the small red circle until the countdown ends, the device should vibrate.
If none of the above helped, try holding the device in your hand and turning it on and off. If you did not feel a vibration while doing that, please contact us via the in-app chat.