Here's some more information:
http://www.garden.org/plantguide/?q=show&id=2102 Is it still in a pot? My guess is that if it is, it has now outgrown it's pot and maybe that's why the leaves have gotten smaller. The above link has some advice about how to prepare a planting site for it.
If it was root-bound when it was planted in the ground, this might also cause a stunting effect. If this is so, you might try to lift it out of the ground and gently unwind the roots. Then make it a larger hole and settle it back in, watering deeply. That might get it going full strength again next season.