Orthographic camera with rotation and ScreenToWorldPoint

I think you have an complex rotation on your camera, like in the one of a isometric game, in this case you should convert your sceen point to ray then raycast it on a plane (look at Plane class) or in the collider geomtry of your game.

Ok Here is how I fixed it in my Game hope this helps. Lassade was correct btw here is how you do it:

You need to spin up a plane so that you can cast a ray at it in order to determine what the distance is from the camera.

So in my game I have a -30 rotation on the X of the camera which is ultimately means the 'world' is further from the top of the screen than it is from the bottom. That means that if I want to shoot a projectile say at the mouse. I need to know the exact mouse click position.

What I did was spin up a Plane in the same position as my world and cast a ray at it to determine what distance it was from the camera.

 // define a plane variable
 // may be Vector3.up in your case depending on your camera orientation
 Plane plane = new Plane(Vector3.forward, Vector3.zero);
 
 // Shoot a ray at it when your event happens, in my case projectile
 
 public void OnCastComplete()
     {
 var projectile = Instantiate(projectile, somePoint.transform.position, transform.rotation);
 
         //Create a ray from the Mouse click position
         Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
         //Initialise the enter variable
         float enter = 0.0f;
 
         if (plane.Raycast(ray, out enter))
         {
             Debug.Log("distance " + enter);
             //Get the point that is clicked
             Vector3 hitPoint = ray.GetPoint(enter);
             //Draw a debug ray to see where you are hitting
             Debug.DrawRay(ray.origin, ray.direction * enter, Color.green);
 
             // create a direction vector (hitPoint => somePoint
             Vector2 direction = new Vector2(
                 hitPoint.x - somePoint.transform.position.x,
                 hitPoint.y - somePoint.transform.position.y
                 );
 
             // addforce force to the projectiles rigidbody in that direction.
             projectile.AddForce(direction * projectileSpeed);
 
         }
 
     }
 

Hope this clears it up a bit. This will certainly help in my game and it took me a long time to find the solution so I hope it helps others.