I find it works the best if you let Sims take it with them to an active career, retail lot they own, flee market stall, or even just for whenever your sim will be outside for a few in game hours in a place with no food.
That way you can actually make them eat it. At school or rabbit hole jobs it's very buggy, and I see no point in that because they always eat at one point during off screen school & work.
Edit: they have automatic lunch breaks so the lunch bags are actually useless there.
But it's strange how your sim came home hungry. They should be eating at school regardless of lunch bags :s
