To keep a sticky note 'stay on top' always in Notezilla , simply click on the pin in the sticky notes top toolbar. It should always stay on top.
With the sticky note that doesn't stay on top, it is possible that you have stuck that sticky note to one or more windows/programs/documents using the 'Stick To Window' option. If a sticky note is not staying on top, go to 'Stick To Window' option from the top left button, click on 'Clear Selection' and then press OK. Now, click on the red pin in order to make it stay always on top.