During some scene (usually for a couple of frames or so only, and usually not during animation), you'll see things in the background that were not there a few frames before or that will disappear (e.g.: A pineapple appearing on table for 2 frames). Clicking on them (very small hitbox if you are playing on mobile) will unlock renders.
Pay close attention to the hints for each render and in which scene they are expected to happen.