Well, I wish I could say I knew "the answer" ... however, not only don't I know it, I don't know all of "them" since your question probably implies several questions at once.

What I *do* know though is that we are lucky to have here one Al, a master of bonsai ... which means he is also a master of all container plants. His username is Tapla. Al not only knows a lot, but he is very patient about explaining it all. Most of my plants are only alive because of him. My photo is up in the plants' post office for sure.

So, first thing to do is read his posts, and I would start with this one:

https://www.houzz.com/discussions/1389927/trees-in-containers-iii#n=221

Although who knows there may be a new version. Ficus questions are very very popular here, and in the House Plants forum. Come to think of it, there may be a specific Ficus in Containers thread too.

Also, see this thread, which has links to some of his best threads:

https://www.houzz.com/discussions/5570006/al-s-tapla-greatest-hits#n=15

Another thing is, if you are really worried, post photos. It's hard to tell what's going on without them.

Also ... there may be nothing wrong. Sometimes they just drop leaves when the light changes.

Well, I thought it was too cold in the house, and I kept the water low and the thing just died.

When we pulled it out of the pot, we realized that the roots were so compacted, they never grew out into the new space! Oh well. Live and learn!