According to a book on bonsai that I recently got, the author said he did the same thing, it took about 3 years before the root system was firmly attached to the rock. What he did was to bury the pot, rock, and all in the garden, (just covering the rock) and dug it up 3 years later, cleaned the soil off the exposed roots, repotted it, and cut away any wires that were not rusted away. A ficus, of course, would not survive the winter unless you are in a warm climate, but it is stillgoing to take a few years for this to happen. Hope you have patience, because that is what bonsai take.