You have a few problems going on with those two pictures.
In the first photo, the strongest lighting is coming from the left of the subject, hence that flash-induced shadow behind her to her right. Assuming you want to stick to your current lighting sou have a couple of options to help reduce that: Either better position your other flash units to fill in that space behind her or do as Wheatfield suggested and redirect some light into those spots.
For picture number two, it's a completely different story. As JimH and Venturi suggested, you can definitely move the chair/subject further away from the backdrop. However, there is this flash-blast above her head that's blowing out that part of the photo and really pronounced shadows to her left and right.
A photo of your setup might help here, but I'm going to assume the shadows are being cast as if you have the other flash units at her head level, to her left and her right and either your on-camera-hotshoe flash pointed above her head or one on a stand blasting from above. You need to try putting the flash units on the sides a bit higher, above her head a bit so the shadows are lower and reduce the intensity of light if you can. That coupled with getting the backdrop farther away from her should help a lot. If you want you can also experiment with a small flash right behind her to light up the backdrop and eliminate or reduce the shadows. All depends on what look you want.
But best of all, you need to play with your equipment, experiment with different positions, angles, heights, and find out what works. Practice truly makes you better