Button Click Only Updates For First Row Selected

I got the database to update, but it only does it for the first row selected not for the others.
Protected Sub btnUpdate_Click(ByVal sender As Object, ByVal e As EventArgs)
For Each row As GridViewRow In GridView6.Rows
' Selects the text from the TextBox
Dim selectedcheck As CheckBox = CType(row.FindControl("chkselect"), CheckBox)
If selectedcheck.Checked = True Then
Dim id As Label = CType(row.FindControl("id"), Label)
cmd.Parameters.AddWithValue("@id", id.Text)
cmd.Parameters.AddWithValue("@compby", txtagent.Text)
cmd.Parameters.AddWithValue("@compdate", lbldate.Text)
cmd.Parameters.AddWithValue("@comments", txtcomments.Text)
cmd.CommandText = "dbo.updatetasks"
cmd.CommandType = CommandType.StoredProcedure
cmd.Connection = conn conn.Open()
cmd.BeginExecuteNonQuery() conn.Close()
End If
Next
End Sub
UPDATE dashboardtasks SET compby = @compby,
comments = @comments, compdate = @compdate
WHERE id = @id;