object follow mouse on x-z plane with angled camera

hi,

i have 3 cameras placed on the scene and user can switch between them. in each of cameras user can drag a wall and place it somewhere that he/she wants.

now the problem is i want the object (wall) to follow mouse on the x-z plane meaning that the transform.z of wall should always be 2f.

the problem is my code works fine on the top camera (all are perspective cameras) but if i switch to other 2 angled cameras, it does not work correctly. these are the position of my cameras :

and this is the code for dragging :

         Vector3 pos = Input.mousePosition;
         Vector3 targetPos = new Vector3 (pos.x, pos.y,gc.activeCam.transform.position.z);
         Vector3 worldPos = gc.activeCam.ScreenToWorldPoint (targetPos);
         worldPos.y = 2f;
         print(worldPos);
         transform.position = Vector3.MoveTowards (transform.position, worldPos, 15f * Time.deltaTime);

(i run this code on update so i use MoveTowards)

what is wrong with my code ?