Make sure the rhizomes are not too deep in the soil. They must be expose to lots of direct sun & air in order to flower. They should barely rest on top of the ground with only the roots going down in the soil. When the sun hits the rhizomes, they send up flower shoots. Don't use mulch or anything.

Mine are exactly the same, not a flower shoot in sight and they are growing in the conditions you describe.

Could they be over-crowded? They should be thinned out every couple of years. Dig them up, discard the old spent rhizomes & give new space to the 'daughter' rhizomes so they can bloom. Don't over fertilize.