When dealing with a pptx file, or another new version of Powerpoint, the situation calls for the Open Office Extension. However, the default settings will convert only the 1st slide of the ppt and the links to other slides (such as "next slide", "last slide") won't work.

To handle all the slides, you need to configure the PowerPointPlugin as follows: besides leaving the openoffice_conversion turned on, which uses the OO Extension, you want to set the convert_to setting to "html_multi". It is defined as "A series of HTML pages, two per slide. One for the slide image, one for the slide text (needs -openoffice_conversion)."

Build the collection with these settings and all the slides in your ppt should have been processed.