Usual reason for them not flowering is that the rhzomes are crowded and/or shaded. They need sun to ripen them to initiate flowering. They are also greedy feeders so they could be hungry. I would suggest digging them up, once the flowering season has finished and getting rid of the old bits from the middle of the plant. Feed the soil and replant with young pieces, with the rhizomes pointing North./South. To avoid the new plants rocking in the wind, cut the leaves back by a half.

Agree with Owdboggy