Ficus burtt-davyi can be a difficult bonsai subject. Sometimes it just won’t grow properly. This tree has been a problem for me over many years.
It has just not been vigorous and I have had to consider discarding it or perhaps re-styling it. My decision was to get radical and to cut the tree in half. One part will become a slant or windswept while the top portion with only one root was secured on a rock to justify its poor root system. Time will tell if I can bring these two to a satisfactory bonsai design.
The overall vigor of the tree may respond to less water, coarser soil and allowing it to rest during the shorter days of winter. Time will tell.