Hi Landy,
Welcome!
Julianh72 and rustigsmed offer some great advice, another way to look at it is when you put the lower number eye piece in the focuser, you are increasing magnification by reducing what is called the Apparent Field of View (commonly referred to as AFoV). So if the object is not exactly in the center of 25mm eye piece, it will most likely be out of view when you put in the 10mm eye piece. And as mentioned, it will also be much dimmer because you have reduced the amount of light coming to your eye.
Re: the barlow, most factory supplied barlows physically move the eye piece away from the focus point you may originally set, towards you as you look at it. To get back into focus, you will need to adjust the focuser in, toward the body of the telescope.
Hope that helps! Good luck!
