Why are the leaves on my rubber tree plant curling under ?

Question

It started off with just one leaf curling under but now all the bottom ones are doing it as well...

Answer 1

"Rubber plants and fiddleleaf figs may loose lower leaves as they mature, especially if the light is too low. Under watering is another cause of leaf drop in rubber plants and fiddleleaf figs. "

http://www.bachmans.com/retail/tipsheets/indoor_plants/FicusforIndoors.cfm

Answer 2

Either lack of water, or too much of...