does not detect knocks on table, but senses shocks while holding in hand

Knock sensor test ky031

const int ledPin = 31; // define ledPin Interface
const int shockPin = 30; // define the percussion Sensor Interface
int val; // define numeric variables val

long lastKnockTimestamp = 0;
int knockTimeout = 2000;

void setup () {
  Serial.begin(9600);

  pinMode (ledPin, OUTPUT) ; // define ledPin as output interface
  pinMode (shockPin, INPUT) ; // define knock sensor output interface

  Serial.println("Running...");
}

void loop () {
  val = digitalRead (shockPin) ; // read digital interface is assigned a value of 3 val
  if (val == LOW) {
    lastKnockTimestamp = millis();
  }

  if (millis() - lastKnockTimestamp < knockTimeout) {
    digitalWrite (ledPin, HIGH);
  } else {
    digitalWrite (ledPin, LOW);
  }
}