For casement windows, the solution may be as easy as cleaning and lubricating the operating hardware with a light oil or dry silicone spray. Avoid lubricants or harsh abrasive cleaners -- these will break down or collect dirt and grime. Gliding window tracks also need periodic cleanings to maintain easy sash operation. Otherwise, you may be looking at a warped frame and sash caused by house settling, which can skew the rough opening the window sits in. Here, we recommend removing the entire window and reframing and squaring the opening. Then simply insert a new window.