Charactercontroller won't go down, only goes up

Hey i have a player with a charactercontroller and an easy movementscript to walk. (player don't need to jump or something like that). but if it walk over a little blok. my player moves up. and is kind of flying above the ground. if go up a stair he keeps flying at that level. but my player won't go down.

sorry for my bad english . thanks for reading

(this is my movementscript)

   public float MoveSpeed = 100f;
   public bool attacking=false;
   public bool IsWalking;
   public CharacterController controller;
   Vector3 lastPos; 
 
   void Start()
   {
     lastPos = transform.position;
   }
 
   void Update()
   {
    if (GameObject.Find("Cha_Knight").GetComponent<PlayerHealth>().currentHealth > 0)
     {
       Vector3 curPos = transform.position;
 
 
       if (Input.GetKey("z"))
       {
         controller.Move(Vector3.forward * MoveSpeed * Time.deltaTime);
         animation.CrossFade("Walk");
         transform.forward = new Vector3(0, 0f, 1f);
         IsWalking = true;
 
         if (lastPos == curPos)
         {
           IsWalking = false;
         }
 
 
 
       }
 
 
 
 
 
       else if (Input.GetKey("s"))
       {
         controller.Move(-Vector3.forward * MoveSpeed * Time.deltaTime);
         transform.forward = new Vector3(0, 0f, -1f);
         animation.Play("Walk");
         IsWalking = true;
 
         if (lastPos == curPos)
         {
           IsWalking = false;
         }
 
       }
 
 
 
       else if (Input.GetKey("q"))
       {
         controller.Move(-Vector3.right * MoveSpeed * Time.deltaTime);
         transform.forward = new Vector3(-1f, 0f, 0);
         animation.CrossFade("Walk");
         IsWalking = true;
         if (lastPos == curPos)
         {
           IsWalking = false;
         }
 
 
       }
 
 
 
       else if (Input.GetKey("d"))
       {
 
         controller.Move(Vector3.right * MoveSpeed * Time.deltaTime);
         transform.forward = new Vector3(1f, 0f, 0);
         animation.CrossFade("Walk");
         IsWalking = true;
 
         if (lastPos == curPos)
         {
           IsWalking = false;
         }
 
 
       }
 
 
       else
       {
         IsWalking = false;
       }
 
 
 
       //animation.CrossFade("Wait");
 
       lastPos = curPos;
 
 
 
 
     }
 
 
   }