Check for dust on the sensor roller. Clean it using compressed air. Also, ensure the yarn is moving perpendicular to the device. Off-angle tension causes fluctuations.
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(if Jie Rui has an official support or downloads page). Check for dust on the sensor roller
The Jie Rui BT-5908 has a three-button control cluster on the right neckband: Jie Rui Bt-5908 Manual Pdf