Dataadapter and Databinding - Update doesn't work

[Wolfgang]

I Created an new Page
I added 3 Textboxes
i created a link button
I added a SQL - Dataaadapter with the wizard
I created a Dataset with the wizard from the dataadapter

in the page_load - Script i add this 3 lines of source
if (!this.IsPostBack)
{this.DB_Connection.Open();
this.DB_VereinDA.Fill(this.dB_VereinDS1);
this.DB_VereinDA.SelectCommand.Parameters["@vnr"].Value="XX"
this.DataBind();
this.dB_VereinDS1.AcceptChanges();
}

i bind my 3 textboxes via properties to the columns of the default view

in the link button i added the line
this.DB_VereinDA.Update(this.dB_VereinDS1);

I start the page with the IE. in the 3 Textboxes there is the text i
expected from the database.

if i press the link button nothing happens (there is no change in the
database)
and if I set a breakpoint and look into the dataset after the update, there
is nothing inside