I was also getting deeply frustrated as my bees were refusing to use the frames to the point that I thought they would swarm through lack of space.
What worked for me was temporarily removing a flow frame from the super and moving up a frame of worker brood (number 3 on Dawns list above). The bees moved up to look after the brood and started working the adjacent flow frame. Yesterday we harvested our first flow frame & all went well :).
I will now use this method with all new flow frames, though obviously the bees need to be bursting at the seams first. Also, the back window shouldn't be used while there is a frame of brood up there or all the bees will fly out.
There is no doubt that the bees took much longer to draw out the flow super out the flow super than the standard box of frames. However, As the sealings on the flow are mostly in place and the box now smells of bees, hopefully it won't take them so long next season...