What is your nectar flow like? Any other hives getting plenty of honey? You may be in a nectar dearth, hard to know without know where you are.
If it was my hive, I would try putting a medium super with traditional frames on top of the Flow super, and see if they used that. If they do, they don’t like the plastic, yet. If they don’t, you are probably in a dearth, for now.