Access database doesn't update after deleting a record

In my simple application, I have attached a access database. In the form there are 3 buttons for ADD,UPDATE & DELETE. My code is this,

Public Class Form1

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    'TODO: This line of code loads data into the 'NamesDataSet.Table6' table. You can move, or remove it, as needed.

End Sub

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Save.Click

End Sub

Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Add.Click
End Sub

Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Delete.Click

End Sub

End Class

All 3 buttons works fine separately. i added several records and updated. then delete 1 record. then tried to update. Unfortunately unable. It gives an error and never update. the error is

Concurrency violation: the UpdateCommand affected 0 of the expected 1 records.

Please tell me how to solve this mess.Thanks!