There is a root eating grub that is very commonly found in the roots and surrounding soil of ficus trees.
It does kill the tree and there is little you can do about it.
Check in the soil close to the roots. These grubs almost look like waxworms ( a little smaller). If you see them throw the whole pot and tree out.
Freeze it if you can to kill the worms. Then I guess go buy a new plant. These parasites are specific to ficus trees so maybe try a scheffelara or hibiscus.
Ficus will drop their leafs if they are put under different lighting. THe new leafs that grow back will be of a colour, ( shade of green), and size best suited to the new light. You can also cause the tree to die if you overwater it. Best to soak the soil , then let it dry out over the period of a week , before the next watering.
When do you get the plant? They tend to go through an adjustment period and drop a ton of leaves at first but then tend to bounce back. The ficus in my veileds cage is over 2 years old and adjusted quite well.
Another hing to consider is to place large river stones at the bottom of the pot AND place the plant on a plant stand which helps it drain properly.