OK, please forgive the cut and paste, but I think have typed this twice today already....
If your bees are not in Flow super, you need 2 things - enough bees and making the plastic frames smell of home. The comments below address both of those:
Only put the Flow super on when all of the following rules are true:
Every frame is mostly filled with fully drawn comb, and
The comb is 80% full of brood, honey or pollen, and
Every frame is well-covered with bees.
The reason is that you need enough bees to defend (from SHB, wasps and wax moths) and use the new space. When the above three items are all true, you have enough bees.
To get the bees to use the Flow super more readily, there are a few tricks. I took 1-2cm blobs of burr comb from an inspection on my hive tool, and just pressed and smeared sideways gently onto the frame faces at about 10cm intervals. You don't need to cover the whole frame, just put enough on to make it smell like the hive. Each frame face ended up with about 12 blobs of smushed wax pressed into it. The bees cleaned it all up within a day or two. You can also paint melted bees wax onto the frames, and there are several other tricks too, which you can search for using the magnifying glass tool at the upper right of the forum window.