The vibration stops when I lock the screen.
This is an iOS limitation — vibration only works while the screen is on. The App keeps the screen awake during a session, and you can use the in-app lock button to prevent accidental touches.
Vibration feels weak.
Make sure the Intensity slider is up and System Haptics are enabled in Settings → Sounds & Haptics. Vibration strength also varies between iPhone models.
Music Sync doesn't react.
Music Sync needs microphone access (Settings → Privacy & Security → Microphone) and works best with music playing out loud near the phone.
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