Parameterized Query Build Error

Why aren't my parameterized variables being added to my Sql query?

I have two parametrized variables set by combobox.text which is selected by the end user.

I get the error below when trying to use a query that uses a parameterized variable.

Additional information: Must declare the scalar variable "@username"

Am I missing something?

Example Query

  SQL = "SELECT stationID, LocationName, plandate, username, status  FROM dbo.joblist WHERE username = @username and status = @status";

Code Snippet

            //Decide what query
            String SQL = SQLSelection();
            //Connection String
            String ConnString = "Data Source=dbsqlexpress; Provider=SQLOLEDB; Initial Catalog=Data; User ID=mobile; Password=PW";
            //Create  and initalize Oledbconnection object and pass connection string into it.
            OleDbConnection con = new OleDbConnection(ConnString);

            //open connection to database
            con.Open();

           //create adapter that sits inbetween dataset and datbase
            OleDbDataAdapter adapter = new OleDbDataAdapter();

            adapter.SelectCommand = new OleDbCommand(SQL,con);
            adapter.SelectCommand.Parameters.Add("@username", OleDbType.VarChar).Value = auditorCmb.Text;
            adapter.SelectCommand.Parameters.Add("@status", OleDbType.VarChar).Value = statusCmb.Text;


            //Create dataset
            DataSet dataset = new DataSet();

            using (DataTable dt = new DataTable())
            {
                adapter.Fill(dt);
                dataGridView1.AutoResizeColumns();
                dataGridView1.AutoSizeColumnsMode = DataGridViewAutoSizeColumnsMode.AllCells;

                con.Close();
                dataGridView1.DataSource = dt;

            int rowCount = rowCount = dt.Rows.Count;
            label10.Text = rowCount.ToString("n0");
            }




        }