I agree with Frenchy's assessment that you may be letting it get too dry.
That said, this plant is a bit on the fragile side and not very tolerant of even minor lapses in watering. In addition, it will never be a big, full, bushy plant. The number of leaves it has now is about all you can ever expect it to have at any one time. As it continues to add new leaves on top, it will continue to lose lower leaves.
