There could be several reasons why they aren't blooming. First, they take a long time to bloom. Some cultivars can take as much as 4 months to bloom from seed. They need full sun - part shade or shade can reduce/prevent flowering. Also, too much nitrogen or too rich of a soil is not good. Over-fertilizing with anything high in N can promote too much foliage at the expense of the flowers. In all cases, it's a matter of patience.
