Code doesn't write characteristic

Question

Hi I'm trying to do a code sample that shows you the heart rate form a polar h6 heartrate sensor on a htc one m8. And I have some problems whine I write the

characteristic.setValue(new byte[]{0x01});
        Log.d(TAG, "x ok");
        gatt.writeCharacteristic(characteristic);
        Log.d(TAG, "Writechar ok");

the log says that the writecharacteristic method was done but the oncharacteristicwrite in the callback doesn't react, Thanks you for your help!