I would say that your bees are busy filling out 2 brood boxes and they are being sustained by the honey frames in these boxes. As Dawn has suggested, there is probably not enough nectar to store in the Flow so they are not bothering.
You will just need to be patient and hopefully a large nectar flow will come (pay attention to the flowering plants & trees in the vicinity, this is your indicator), its really dependent on how much forage is available. I only ever use single brood boxes and for the Flow super I waited until the bees filled every frame and started building comb on the roof, then I knew they were ready for the flow super. Some questions to ask yourself before adding a Flow super:
- Do you have bees on every frame in your hive?
- Are all the cells occupied in all those frames in your 2 brood boxes with either brood, pollen or honey
- Are your bees depositing nectar into cells where brood should be?
- Are your bees building comb in between the upper and lower brood boxes or on the roof?
If so, then they are ready for a Flow super.
And in addition to this, see the HoneyFlow FAQ for some other tips.
How to encourage bees to fill the Flow Frames