This definitely sounds odd. I completely understand how this would disrupt your workflow. First thing worth ruling out: skipped slides are intentionally left out of exports. If a slide has a closed-eye icon on its thumbnail, it'll be excluded. You can right-click any slide → Unskip slide to bring it back. Full details here: Skip or unskip slides
If none of your slides are skipped, this may be something else. Could you share a few details, so we can investigate further?
What format are you exporting to (PDF, PPTX, PNG)?
Are you on the desktop app or browser, and which OS?
Are you trying to exporting all slides or are the missing slides missing when you’re choosing a specific selection?
Do the same slides go missing every time you re-export?
Let us know, and we’ll continue troubleshooting this together.