Try this in the loop() and let me know what happens.
analogValue = analogRead(analogPin); // read the analog input (value between 0 and 1023)
if(analogValue > 700) {
servoMotor.write(0); // Spin servo at max speed clockwise
} else {
servoMotor.write(90); // Stop the servo
}
delay(15); // waits for the servo to get there