There are two main reasons for hardy geraniums failing to flower prolifically. An extended summer drought may slow growth and reduce flowering, and can also cause the lower leaves on the stems to drop off. Regular irrigation in dry periods is the answer here.
There is another possibility. Sun-loving varieties will flower poorly in constant shade, especially overhead shade from trees. It may be that when your hardy geraniums were planted, they were in good light, but that nearby trees have expanded their growth. Of course, they may have been planted in the wrong place to begin with. Certain cranesbill plantings work best with south-facing garden ideas or sunny container spots, so bear this in mind. Depending on the cranesbill variety you are trying to grow, some are likely to flower well with plentiful sunshine.