Login Control UserName Only Accept the Initial Test Value of UserName and Password, Can't Test Any other set

Hi,

I am a beginner of .Net Web Developer.

I programmed a simple Web app in VB.NET 2005 to let an user log in at first, and the UserName will be passed to the next page for displaying his or her relevant information .

the Login Control connects to a data table in SQL Server 2005 for log in authentication. I used UserID stored in the database as value of UserName text field of the Login control.

The problem is, during my many tests, only the very first UserID and password (the password is actually an user's last name) got successfully log in, no matter how many time to repeat to use this set of value. Except this set of value, the Login process was never succeed when using any other pairs of UserID and LastName, even I ensured to type in correct value in the text fields.

Your advise will be very appreciate!

Part of my code: -----------------------------------------------------------------

Private Function YourValidationFunction(ByVal UserName As String, ByVal Password As String) As Boolean


        Dim boolReturnValue As Boolean = False
        Dim strConnection As String = "Data Source=myServerName;Initial Catalog=myDatabaseName;Integrated Security=True;"
        Dim sqlconnection As New SqlClient.SqlConnection(strConnection)
        Dim SQLQuery As String = "SELECT distinct [UserId],[Last name] FROM myUserTable"
        Dim command As New SqlClient.SqlCommand(SQLQuery, sqlconnection)
        Dim Dr As SqlClient.SqlDataReader
        sqlConnection.Open()
        Dr = command.ExecuteReader()
        While Dr.Read()
            If (UserName = Dr("UserID").ToString()) And (Password = Dr("Last name").ToString()) Then
                boolReturnValue = True
            End If
            Dr.Close()
            Return boolReturnValue
        End While
        Return boolReturnValue
    End Function