The two most common reasons for Geraniums not blooming prolifically are too little light or too much fertilizer.
Geraniums are a sun loving plant that need 4-6 hours of full sun a day, or perhaps longer in somewhat filtered light. South and west exposures are usually best. In too little sun, the plant will often grow, but will tend to grow a little bit taller or leggier, as it reaches for the light and it definitely will not bloom as much. The number of flowers is very much related to the amount of sun that the plant gets. As far as fertilizing goes, if Geraniums, like most annuals, get too much, you will get large, lush green plants and not as many blooms because the plant will be in more of a vegetative mode. In containers, if you feed your Geraniums, every 3 to 5 weeks, Use any of the organic fertilizers.