CheckChanged event not firing

i have a checkbox and a button on a page.Button is not enabled

i want to enable the button when checkbox is checked

i have this code to enable the button

Protected Sub CheckBox1_CheckedChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles CheckBox1.CheckedChanged
        If DirectCast(sender, CheckBox).Checked = True Then
            Button1.Enabled = True
        Else
            Button1.Enabled = False
        End If
    End Sub

it seems that checkox1.checkchanged event is not firing