Hibiscus Won't Flower?

I have a hibiscus (not sure what kind) in a pot beside a very sunny window and for the last year it has been constantly producing an incredible number of flower buds, but they all fall off at the stem before they develop. I did have a pest problem, but they were under the leaves (no trace in the buds) and now are gone. Since getting rid of the pests and the arrival of spring the leaves have started turning yellow and dropping off. I keep the soil moist (not wet) and fertilize every other week. The photo shows yellow leaves, new growth, and bud preparing to drop. Why is this plant growing and budding like crazy, but not able to flower and losing its leaves?