Could you please provide more information how it looses its leaves? Does it experience extreme heat periods? Does it experience long periods of drought? Is it being affected by fungal issues such as black spot? Could some herbivore be eating the leaves? Many plants will experience "summer dormancy" (shedding of foliage) in response to periods of extreme heat. During those periods, they transpire more water than the roots can absorb, so in an effort of self preservation, they will shed the foliage to stop the movement of water up the stems and out the petioles (pores on the undersides of the leaves) to prevent too much water loss. Periods of extreme drought will result in similar results. Good luck in figuring it out.