PreFabs not colliding with other GameObjecs, but collide with themselves.

i'm fairly new to unity and currently in process making my first game using unity.

i have currently a player(GameObject: sphere) revolving around a centre point in circular motion.

Another enemy, it is a prefab and i instantiate after a random amount of time and move it towards the centre around which the player revolves.

code for the instantiation of the PreFab enemy.

 using UnityEngine;
  using System.Collections;
  
  public class enemy_movement : MonoBehaviour {
  
  
         public float targetTime;
          int i=0;
          int num=-1;
          int temp;
          public Transform[] patrolpoints;
          public Transform planetposition;
      public int movespeed;
      private int currentposition;
      public int size;
      GameObject[] enemy=new GameObject[500];
       GameObject PreFab;
    //  public GameObject deathparticles;
      
      // Use this for initialization
      void Start () {
      
      targetTime=random_value_float();
      PreFab=(GameObject)Instantiate(Resources.Load("enemy"));
      PreFab.name="enemy";
      //currentposition=0;
      }
      
      
      
     
     
 
      
      
      // Update is called once per frame
      void Update () {
      
      targetTime -= Time.deltaTime;
      
      if((targetTime<=0 && num<=1))
      {
      num=num+1;
      temp=random_value();
      
      //this is the error line
  
  
      enemy[num] = (GameObject)Instantiate(PreFab, patrolpoints[temp].position, Quaternion.identity);
      enemy[num].name="enemy";
      targetTime=random_value_float();
      
      }
      
          for(i=0;i<=num;i++)
          enemy[i].transform.position=Vector3.MoveTowards(enemy[i].transform.position, planetposition.position , movespeed*Time.deltaTime);
      }
      
      
      
      
      
      
      int random_value()
      {
      int num = Random.Range(0, patrolpoints.Length);
  return num;
      }
      
      float random_value_float()
      {
      float num = Random.Range(5, 30) * .1f;
  return num;
      }
     
  }

followed by the code of the player, that detects collision with the enemy prefab.

 using UnityEngine;
 using System.Collections;
 
 public class Playermovement : MonoBehaviour {
     
     public Transform playerpos;
     
     int flag=0;
     float angle =0;
     float speed=(4*Mathf.PI)/5 ;//2*PI in degress is 360, so you get 5 seconds to complete a circle
     float radius=1.8f;
     float x;
     float y;
 
     // Use this for initialization
     void Start () {
     
     }
     
      void OnTriggerEnter(Collider collision) 
     {
     Debug.Log ("Called by "+ transform.gameObject.name);
     
    if(collision.gameObject.name == "enemy")
       print("player hit " + collision.gameObject.name);
       
        //else 
      //{
       //   Debug.Log("Hit something else!"+ collision.gameObject.tag);
     // }
 }
     
     // Update is called once per frame
     void Update()
  {
     
       if(flag==0)
       {
       angle += speed*Time.deltaTime; //if you want to switch direction, use -= instead of +=
       //flag=1;
       }
       else if(flag==1)
       {
       angle -= speed*Time.deltaTime; //if you want to switch direction, use -= instead of +=
       //flag=0;
       }
       
      if (Input.GetKeyDown("space"))
     {
     if(flag==0)
     flag=1;
     
     else if(flag==1)
     flag=0;
     }
      
      x = Mathf.Cos(angle)*radius -1.06f ;
      //x=x-1.12;
      y = Mathf.Sin(angle)*radius -1.04f;
      //y=y-0.56;
      
       transform.position = new Vector3(x, y, 0);
       
       
            // print("up arrow key is held down");
  }
  
  
 }
 

now the issue

the enemy prefabs (that are instantaited) can collide with each other but not with the player.

if i place the enemy near the player (without instantaiting it) it gets collided with the player .

player is a sphere with isTrigger checked and Rigidbody component added,same goes for the enemy PreFab. and have tried both names and tags , nothing works.

any advice would be appreciated.

thanks!!