I noticed that too. Some lots from gallery that have been run as retails (have displays set, and items for selling are assigned) will work. But if the lot was uploaded "fresh" - right from the build mode without going through life mode and working as business - will be empty.
What i think can solve this - you can cheat. For example if you've build your own retail - but you don't want to run it with your active family - go to manage world, choose a random family, buy that retail store, set up displays and assign prices then switch back to the family you played before.
I myself didn't try it - but i'm pretty sure it will work...logically thinking:))
From my understanding npc stores don't work because there was nothing assigned for sale