If you are fertilizing with anything that is high in nitrogen, supplement with phosphorous. Nitrogen is great for leaves and stems, but not for flowers. Flowers need phosphorous. You can try something more natural like bonemeal or a product like superphosphate to add to your current fertilizer or switch to synthetic fertilizer like Miracle Gro bloom plus.

Be sure your plant is getting enough sunlight.  Many plants and trees require at least part sun (about 6 hours of full sun) in order to bloom well.

Here is a link that may help as well: