I am thinking your plant is still acclimating to your location. My scheffleras grows outdoors here in a big container, part sun/part shade, in a west/southwest orientation protected by the shade of city trees and humidity here is quite dismal. It is quite drought tolerant too. That's is why I like it..that is all we have here, dry heat and more heat, and it thrives well. I do have to water it more often in summer, had to adjust watering as season changes.
The container does seem tight, but as everyone suggested check the roots, and you may want to separate one of them, if there are several there already.
What is the orientation of that window? Can you move the table a bit closer to the window?
