Select "Slide Show" | "Set Up Show..." menu item. In the "Show slides" box,
check if "All" is the selected option. If it is something else, then the
slide show would show only that set of slides.
Also, check if the slides are set to be hidden. In the Slide Sorter View
("View" | "Slide Sorter" menu item), see if any slide has a cross on its
number. If it has then that slide is hidden and you will not see it normally
during slide shows. Unhide it by selecting the slide and clearing the check
against "Slide Show" | "Hide Slide" menu item by selecting it.
- Chirag
PowerShow - View multiple PowerPoint slide shows simultaneously
http://officeone.mvps.org/powershow/powershow.html