I have purchased this Ficus last year.

I have repoted it last winter in a mix of:
4 parts of bark,
3 parts of Akadama,
2 parts of Se-T (like pumice stuff from Kaizen Bonsai)

I have found it work very well in my previous place (very worm and sunny apartment). I had to water it twice a day which means it had good drainage. Picture taken after repoting below.

… and then we have moved with my girlfriend to a little house around May. On the beginning everything was fine; the tree was placed in similar to previous place at the huge window but this time at the ground level floor, not this worm as previous one (some old trees are blocking the sunshine for some part of a day). I found the soil being dump and wet for much longer than before and adjusted my water routines to the tree requirements – I THOUGHT….

Suddenly it has started throw the leaves; they were turning yellow first. It was worst and worst so I have cut off watering and feeding as it was clear to me that it has problem with roots and water absorbing. The scariest thing was that the soil stayed dump for over a week without watering! I have decided to act and replaced top layer of soil with a mix I used this year for my outdoor deciduous trees (3 parts akadama, 1 part kiryu, 1 part Se-T, 1 part bark) with 1 extra part of Se-T to keep the mix dry. I did not touch the roots, just took everything that was loose above them.
This sort of slow down yellowing the leaves and engaged new leaves but even that new and young ones were getting yellow; few of them every day.
I have started to read my books and I have found something interesting in ‘Ficus: The exotic bonsai’ book by Jerry Meislik. He is suggesting to use plastic (bag) to cover new graft; it helps to keep it moist.
This made me think. It was obvious that roots stopped to grow and they not transporting water up the trunk of the tree. On the other hand – this is ficus and as you can see on the picture it can creates aerial roots very easily which means it (probably) can take water literally from the air….
And I have cover entire tree in a huge plastic bag.
… so now, after few weeks of being almost completely leafless my ficus looks like this

It is still a bit leggy but I am going to let it grow freely.

I think that the combination of well drained soil and a plastic bag is very good cure for ficus leaf drop.

Ps. I went even one step further… I am using plastic bag technic to create additional aerial roots as I have found many new tiny ones on the tree after ‘plastic bag’ therapy.