A couple of thoughts for you.
- Bees generally (not always) like to extend brood downwards and store honey above. That is why the Warre hive system works. Although they will walk across the ‘nadired’ (not supered) Flow frames, they may not use them for honey.
- You need to think about your hive’s drones. How will they get out? Do you intend to make an upper entrance for them?
- Are the Flow supers new? Have you put any wax on them?
- Are the hives stuffed full of bees? They won’t use the super until there are enough bees.