I cant see new element in Datagridview without restart

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C#

DataGridView

I have added a record to the underlying db and after I add the record i do a datagridview.Refresh(); and i dont see the newly added record.

If i stop and start the application its there. What am I doing or not doing?
Notes : button1 and datagridview is in different Forms. I made datagridview's Modifiers public.
This project is Windows application.

public class CustomerService
{
    public List<Customers> ShowAll()
    {
        List<Customers> customers = new List<Customers>();
        SqlConnection conn = new SqlConnection("data source=.; database=custer; user id=sa; password=*****");
        SqlCommand cmd = new SqlCommand(" select * from Customers ", conn.Open());
        SqlDataReader dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            Customer customer = new Customer ()
            {
                CustomerID = dr.GetInt32(0),
                CustomerName = dr.GetString(1),
                CustomerSurname = dr.GetString(2),
            };
            customers.Add(customer);
        }
        conn.Close();
        return customers;
    }
  }

    private void button1_Click(object sender, EventArgs e)
    {
        CustomerService service = new CustomerService();
        if (txtCustomerName.Text != "" || txtCustomerSurname.Text != "")
        {
            musteriservice.MusteriEkle(txtCustomerName.Text, txtCustomerSurname.Text); //this rows is other method .I am using for adding new customer 
            MessageBox.Show("Customer Added");
            Form1.dataGridView1.DataSource = service.ShowAll();
            Form1.dataGridView1.Refresh();
        }
        else
        {
            //……………
        }
    }    }