Continuous moving ball stuck at some time

I am working on 2d game where monster with circular body moving continuously on screen. I have established grid which get filled by player movement so overall area for monster gets reduced based on player cover its area.

Using following code monster is moving continuously but at some point it get stuck at some corner and stop moving.

 public class EnemyMovement : MonoBehaviour
 {
     private bool isStartMoving;
     private Vector3 direction;
     private float sFactor = 10.0f;
     private float localScaleX, localScaleY;
 
     //
     public float cSpeed = 5.0f;
     
     void Start ()
     {
         InitializeValues ();
     }
 
     private void InitializeValues ()
     {
         localScaleX = transform.localScale.x;
         localScaleY = transform.localScale.y;
         float xDirection = Random.Range (0, 2) * 2 - 1;
         float yDirection = Random.Range (0, 2) * 2 - 1;
         direction = new Vector3 (xDirection * cSpeed, yDirection * cSpeed, 0f);
 
         rigidbody2D.velocity = direction;
         isStartMoving = true;
     }
 
     void FixedUpdate ()
     {
         if (!isStartMoving) 
             return;
 
         // current velocity
         Vector3 cVel = rigidbody2D.velocity;
         if (cVel == Vector3.zero) 
             return;
 
         // normalized vector * constant speed
         Vector3 tVel = cVel.normalized * cSpeed;
         if (tVel.x > 0) 
             tVel.x = cSpeed;
         else 
             tVel.x = -cSpeed;
 
         if (tVel.y > 0) 
             tVel.y = cSpeed;
         else 
             tVel.y = -cSpeed;
 
         rigidbody2D.velocity = Vector3.Lerp (cVel, tVel, Time.deltaTime * sFactor);
     }
 }

I have assign physics material in following manner.

Please give some guidance in this.