From Cedar - If the brood box is packed and there isn’t any action on the Flow comb at all, try pressing some burr comb or wax foundation into some of the Flow combs. The bees will reuse the wax on the Flow comb. I imagine that once they take to it for a first time you shouldn’t have any trouble after that.
I hope you get some results before the season is out.
This Flow user had no problems… WOW, WOW and WOW! :)
The other info that was mentioned - but it looks like you have covered is -
Have you pulled out any of the Flow frames to see if they are starting to put honey in the centre? Or have they waxed any cell joins? Did you set the Flow combs into the cell closed position with the key before installing? Are all of the frames in the brood box drawn and full of brood, pollen or honey yet?