Scheflera need bright, indirect light. Therefore, if/when outside, shade it between 11:00 AM and 4:00 PM. A few hours of early morning or very late afternoon sun is great. For me at least, they did best when I kept the potting soil constantly moist. I also frequently fertilized my plants. Think: "Weakly-Weekly". With a large plant, it is common for the older, lower leaves to drop. These large plants can be sort of messy.
These plants are vigorous growers and the roots tend to fill the pot very rapidly. Your plant may be root-bound but the only way to tell is to remove the plant from the pot (if you haven't already done so). Just hit the side of the pot with the heal of your hand, rotating the pot as you do so. This will loosen the root-ball and the plant should simply lift out easily. If the plant is root-bound, with the roots thick and beginning to circle the inside of the pot, you need to re-pot into a pot that is 4-6" greater in diameter. If the roots are circling the ball, tease them out so that they can grow more naturally, down and out. You want a well-draining potting soil but you don't want the soil to remain sloppy-wet. An open, well-draining soil can remain moist without drowning the plant.