Unable to populate multiple DropDownList on same page using same stored procedure in asp .net

I am trying to populate multiple DropDownList on same page using same stored procedure in asp .net

Following is my code:

1) Page Load Event

if(!Page.IsPostBack){
    fillDropDown(SearchByBrand);
    fillDropDown(SearchByBrand1);
}

2) fillDropDown Method

protected void fillDropDown(DropDownList list) {
    try
    {
        cmd = new SqlCommand("fillDropdownWithBrandName", conn);
        cmd.CommandType = CommandType.StoredProcedure;
        reader = cmd.ExecuteReader();

        list.DataSource = reader;
        list.DataTextField = "BrandName";
        list.DataValueField = "BrandID";
        list.DataBind();
        list.Items.Insert(0, new ListItem("--Select Brand--", "0"));
    }
    catch(Exception e){
        errorMessage = e.StackTrace;
    }
    finally{            
        cmd.Dispose();
        conn.Close();
    }
}

3) DropdownList That I have used 1st DropDown

<asp:DropDownList ID="SearchByBrand" CssClass="form-control" runat="server">
</asp:DropDownList>

2nd Dropdown list is in UpdatePanel which is further inside bootstrap modal which gets open on button Click.

<asp:DropDownList ID="SearchByBrand1" CssClass="form-control" runat="server">
</asp:DropDownList>

Stored Procedure

ALTER PROCEDURE fillDropdownWithBrandName
AS
BEGIN
SELECT BrandID,BrandName FROM Brand
END

Only One DropdownList Gets populated. What could be the problem?