OnTriggerExit not working

I have a code that has a platform drop when the player leaves it. But, at the code never calls, and the player just falls through the platform instantly. My code is

 void Start () {
     rb = GetComponent<Rigidbody>  ();
 }
 void OnTriggerExit(Collider other){
     if (other.gameObject.CompareTag ("player")){
         rb.useGravity = true;
     }
 }

any fixes?