I had the same issue last year. I have 2 hives, side by side, both with 2 brood boxes very healthy. I put the flow boxes on about a month ago and one I will harvest today (fully capped and every frame full) but the other hive hasn’t barely touched it. The is a few cells with some wet honey but very few. They put a little in the flow frame then bring it back into the brood boxes. The brood boxes are full without any more room to put honey so I’m not sure why they won’t start using the flows. I had the same issue last year with this hive and ended up getting no honey all year from the flow frames. Should I give up and put a regular honey super on it or try something else? Any advise would be appreciated!