Grove GSR sensor unable to adjust resistor to 512, max it goes to is 288

Hi,

I am new to this, I got the sensor few days ago.
For some reason I am not able to adjust the sensor above 288, I can go below 288 but not above.

The code I used on the IDE is

const int GSR=A0;
int sensorValue=0;
int gsr_average=0;

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

void loop(){
long sum=0;
for(int i=0;i<10;i++) //Average the 10 measurements to remove the glitch
{
sensorValue=analogRead(GSR);
sum += sensorValue;
delay(5);
}
gsr_average = sum/10;
Serial.println(gsr_average);
}

I am not sure what I am doing wrong.