Answer: Cytisus (sweet broom) requires full sunshine and dry soil to perform well. If your broom isn't getting adequate sunshine, or is getting too much water, bloom will be sparse. Move your plant to a sunnier spot and provide very little water this season. It should flower as soon as it adjusts to its new home.
Brooms are generally disease and insect pest free so I can't imagine what kind of caterpillars were feeding on the plant last year. It's usually not a good idea to spray insecticides on plants until you've positively identified the pest (caterpillars can be immature butterflies!).
Q&A Library Searching Tips