All of the puzzle pieces needed to complete a puzzle should be available to you but some may have been pushed outside the visible area on the table. Try zooming all the way out to bring more of the puzzle table into view. Pinch to zoom in and out or if you are using a mouse with a scroll wheel then roll the wheel to control the level of zoom.

To reach to far outside edges of the puzzle table to grab a stray piece you may need to zoom all the way in then slide the puzzle table to reach these edges.