Can't Get gravity function to work

I want to have one “particle” attract other particles to it. So to test it, I created two spheres. One of which I added the following code to:
[RequireComponent(typeof(Rigidbody))]
public class Attraction : MonoBehaviour
{
public float pullRadius = 2;
public float pullForce = 1;

    public void FixedUpdate()
    {
        foreach (Collider collider in Physics.OverlapSphere(transform.position, pullRadius)) {
            // calculate direction from target to me
            Vector3 forceDirection = transform.position - collider.transform.position;

            // apply force on target towards me
           collider.GetComponent<Rigidbody>().AddForce(forceDirection.normalized * pullForce * Time.fixedDeltaTime);
        }
    }
}

But what ends up happening is nothing. I tried making the radius of pull greater, and the other sphere wouldn’t move toward the initial one, and I have no idea why. It just didn’t move. Help!

Here this should help, I normally go around it by finding Rigid body’s in a sphere however to keep it simple below should work,(using deltatime instead of fixeddeltatime)

using UnityEngine;

public class Gravity : MonoBehaviour {

    //Create Public Variables
    public float pullForce = 2f;
    public float pullRange = 10f;

	void Update () {
		//First lets find all colliders in the shere
        foreach(Collider coll in Physics.OverlapSphere(transform.position, pullRange))
        {
            //Get the Direction
            Vector3 pullDirection = transform.position - coll.gameObject.transform.position;

            //Aplly the force onto the object but multiply it by delta time instead of fixeddeltatime
            coll.gameObject.GetComponent<Rigidbody>().AddForce(pullDirection.normalized * pullForce * Time.deltaTime);
        }
	}
}