为什么鼠标点下飞机还是动不了

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Plane : MonoBehaviour
{
public bool animation = true;
public int frameCountPerSecond = 10;
public float timer = 0;
public Sprite[] sprite;
public SpriteRenderer spriteRender;

public bool isMouseDown = false;
public Vector3 lastMousePosition = Vector3.zero;
// Use this for initialization
void Start()
{
    spriteRender = this.GetComponent();
}

// Update is called once per frame
void Update()
{
    if (animation)
    {
        timer = timer + Time.deltaTime;
        int frameIndex = (int)(timer / (1f / frameCountPerSecond));
        //print (frameIndex);
        int frame = frameIndex%2;
        spriteRender.sprite = sprite[frame];
    }
    if (Input.GetMouseButtonDown(0))
    {
        isMouseDown = true;
    }
    if (Input.GetMouseButtonUp(0))
    {
        isMouseDown = false;
    }
    if (isMouseDown)
    {
        if (lastMousePosition != Vector3.zero)
        {
            Vector3 offset = Camera.main.ScreenToWorldPoint(Input.mousePosition) - lastMousePosition;
            this.transform.position = this.transform.position + offset;
            checkPosition();
        }
        lastMousePosition = Camera.main.ScreenToWorldPoint(Input.mousePosition);
    }
}

看逻辑是，鼠标点下并且移动，飞机才跟着走呀。