firstly when things aren't flowering, i would check the soil to start with, are you fertilising? if yes What!? ok if its blood and bone or a similar organic product, perhaps your tree is lacking something for fruiting and flowering? If you think this is the case try getting some sulphate of potassium or potash you should be able to get this for your locale nursery, if not the smallest amount of wood ash sprinkled around the base will work, and i mean SMALL as too much will finish it off. Does it look healthy other than its lack of flowers? i wouldn't go hard with the fertiliser if its unwell that too will kill it. hope this helps.