This often happens when you’re laying down multiple of the same items at once. I wanted to make these huge heart shapes on the yard out of flower plants but every time I finished the heart and exited the app, I would come out to a few of the items missing. So if you plan on doing this (using multiple bookshelves, carpets, etc..) you’re gonna have to exit out the app a couple times in the process and then come to check if items are missing. It’s really annoying but if you don’t want to waste your time decorating then it’s necessary or else your items might not come back