When creating slides in PowerPoint, the layout determines the underlying structure
of each slide. This includes headings and reading order. Starting with a blank slide
and adding items can create an out-of-order structure with only some content visible
to a screen reader.
In order to create the correct structure on each slide, select one of the preset layouts
in PowerPoint. You can choose a layout when creating a New Slide, or by selecting Layout from the Home tab. Just remember not to select the Blank option.
If none of the of the existing layouts meet your needs, use the Slide Master to make adjustments, rather than just adding items to the slide.