I am trying to detect LPG leakage by using MQ4 gas sensor. I have made this circuit

My Arduino code is

int sensorValue;

void setup()
{
  Serial.begin(9600);      // sets the serial port to 9600
}

void loop()
{
  sensorValue = analogRead(0);       
  Serial.println(sensorValue, DEC);  
  delay(100);                        
}

On my serial monitor, I am getting values like

307
277
289
282
283
283
260
248
265
285
308
278
278
315
321
316
327

These values are not getting change even after putting sensor near gas lighter. Did I made any mistake in circuit or this sensor could be faulty ? Any help is appriciated.