A "watering regimen" is not ideal and might be (part of) your problem. Over watering will sometimes cause leaf drop of apparently healthy leaves. Most of the time, our plants need to be watered at different times, based on the many variables we encounter in our hobby. What kind of soil is it in? What type and size of pot is it in? Did you repot this year? Are there 2 different types of soil in the pot (organic nursery soil and inorganic bonsai substrate)? What I'm getting at is that if pests and pathogens are not apparent, it's probably a root problem. I'd suggest lifting the plant out of its pot (carefully) and inspecting the roots (superficially), looking for pests, soggy soil, foul odor, etc.