Try shooting with the camera sideways (Portrait instead of landscape) so the on camera flash/ mounted flash reaches around the shadow side. just to see if it helps
SUN/WINDOW...........
(Subject)........... SHADOW - then Flash this side
Or get some tin foil (Not sure what it is called in the US, perhaps aluminium foil) and try to use it to 'bounce' light/flash around to fill the shadows.
I'd practice with a figureen or Cup or something then upscale it for your actual use.
Apologies if you've already tried this.