Box collider (with Rigidbody attached) gets stuck into another Box Collider

Hi there, I'm having some problems by moving a box collider with a rigidbody attached to it through another Box Collider, just like a Player moving on top of a platform, the problem is that it gets like inside of the platform collider, it's strange and 'cause of it it slows the Player instead of adding speed, I did search for a solution but I didn't find anything. There's an image of what I'm saying:

And there's my own movement script (still in development):

 using UnityEngine;
 using System.Collections;
 
 public class PlayerMovement : MonoBehaviour {
     public float normalSpeed;
     public float maxSpeed;
     public float jumpForce;
 
     public bool isOnGround;
 
         // Use this for initialization
         void Start () {
         
         }
         
         // Update is called once per frame
         void Update () {
         print(rigidbody.velocity.magnitude);
         if (rigidbody.velocity.magnitude > maxSpeed)
         {
             rigidbody.velocity = rigidbody.velocity.normalized * maxSpeed;
         }
         if (Input.GetKey("left"))
         {
             rigidbody.AddForce(-normalSpeed, 0, 0);
         }
         else if (Input.GetKey("right"))
         {
             rigidbody.AddForce(normalSpeed, 0, 0);
         }
         if (Input.GetKeyDown("up"))
         {
             if (isOnGround == true)
             {
                 rigidbody.AddForce(new Vector3(0, jumpForce, 0), ForceMode.Impulse);
                 isOnGround = false;
             }
         }
         
         }
 
     void OnCollisionEnter(Collision collision)
     {
         if (collision.collider.tag == "ground")
         {
             isOnGround = true;
         }
     }
 }

I'll be looking for answers, and possibly a solution. Thankyu.