Why is my raycast randomly not registering?

I'm making a top down shooter, I am using a raycast to find out if I can shoot an enemy. I am testing on a still cube. If I am standing in a certain area it works, but other areas seem to break the script. CAn someone help me. using UnityEngine; using System.Collections;

 public class Player_Movement : MonoBehaviour {
 
     public int gunRange = 10;
     public float speed = 10;
     public Transform lineStart, lineEnd;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void FixedUpdate () {
         //Aim player at mouse
         //which direction is up
         Vector3 upAxis = new Vector3(0,0,-1);
         Vector3 mouseScreenPosition = Input.mousePosition;
         //set mouses z to your targets
         mouseScreenPosition.z = transform.position.z;
         Vector3 mouseWorldSpace = Camera.main.ScreenToWorldPoint(mouseScreenPosition);
         transform.LookAt(mouseWorldSpace, upAxis);
         //zero out all rotations except the axis I want
         transform.eulerAngles = new Vector3(0,0,-transform.eulerAngles.z);
 
         Debug.DrawLine (lineStart.position, lineEnd.position);
         //moves player forwawd
         if (Input.GetMouseButton (0)) {
             transform.Translate (Vector2.down * Time.deltaTime * speed);
         }
         //test to see if hitting enemy
         if (Physics.Raycast (lineStart.position, lineEnd.position) && Input.GetMouseButton(1)) {
             Debug.Log ("Shot enemy");
         }
     }
 }