When you consider that it’s only in the forthcoming changes to strata law that developers will be required to provide all documentation to the owners corp, you can see the potential for paperwork to go missing.
I was peripherally involved in a legal dispute once where the developer refused to let the owners see the plans because they were ‘copyright’ (the owners needed to see them so they could pursue a defects claim).
So don’t be surprised that the plans for your building aren’t immediately available but it may take some detective work on your part. Just trust that someone, somewhere has a copy – your problem is who?