How to make servo rotate completely?

I wrote some code that makes my Servo motor rotate whenever the photoresistor gets dark. I held my photoresistor up to a white computer screen and periodically made the screen black. When it sensed black, the servo would move, but when I flash the black color very quickly, (for less than a second) The servo would rotate, but just barely. How do I make it so that the servo rotates completely to how much it should and then go back when it sees white? I am including my current code below.

#include <Servo.h>
Servo myservo;
int sensorPin = A0;
int sensorValue = 0;
int val;
void setup() {
  // put your setup code here, to run once:
void loop() {
  // put your main code here, to run repeatedly:
  sensorValue = (analogRead(sensorPin));
  if (sensorValue < 650) {
    // cactus 
    val = 130;
  else {
    //no cactus
    val =  80;
  Serial.println(sensorValue); //debug

Servos take time to move. A typical servo is specified at 300ms to move 60 degrees (unloaded). Is the servo loaded?

Give the servo enough time to move. Introduce a bit of delay when it senses the cactus. Use millis() for the timing, not delay().

Non-blocking timing tutorials:
Several things at a time.
Beginner's guide to millis().
Blink without delay().

Or you could try VarSpeedServo.h instead of Servo.h. That has a 'wait' parameter on the write() that should allow the servo complete the move before continuing. I've used that library a few times but not for that purpose so you'll have to try it yourself to see if it does what you need.