You must have two things for a footer / page number to show:
1. A placeholder
2. Populate the placeholder
To ensure you have the placeholder on each master -- AND slide layout -- go
to View | Master | Slide Master. Check every slide thumbnail to ensure there
is a placeholder for the slide number (and for the footer and the date, if
desired). If these are missing on any layout, copy and paste the
placeholders from another slide master or layout.
Then go to Header and Footer (on the View menu in PPT 2003 and prior, on the
Insert tab in PPT 2007 and 2010) and make the appropriate selections.