Excluding any insect or disease problem, I suspect the browning has something to do with watering. They like to be kept on a slightly moist side but not overwatered with soggy soil. Browning can also be due to low humidity. China Doll plants are temperametal, but since you've had it for 2 years, it must like the location. Browning tips can also happen with salt buildups - be sure to leach the soil medium when you do water. Pour enough to soak the soil and drain out the bottom drainage holes (hopeully there is drainage). Any water remaining in the saucer or pot below the container should be poured off.
