Ficus do tend to drop the leaves in shaded areas. They self-prune to reduce the number of leaves that are not "contributing" energy for the plant. Ficus also drop leaves when you change their location, level of light, or because they are kept too dry or too hot. Most of them are tropical species. F. benjamina can be fussy!
A couple things to check:
Is the potting soil constantly soaking wet? Increase drainage by enlarging the pot drain holes, putting some pebbles in the bottom, adding perlite to the lower layers of the soil, or even packing peanuts. Soggy roots will eventually suffocate and develop rot and molds.
If too much of the plant is shaded, you can put a regular tube fluorescent light somewhere along the side of the cage vertically so more of the tree gets light. It may help regrow leaves. Sometimes the tree never does bush out again, but pruning the dead twigs (remember to wipe off the fresh sap) may trigger new growth. I use my "old" ReptiSuns. Get more use out of them and the plant doesn't really care about the UV level. Some people have 2 trees and rotate them outdoors or into a sunny window to help them along.
Are you feeding or overfeeding the tree? Ficus can handle pot binding, but if there are roots growing out of the pot drain holes it is time to repot it in a larger one.
