Hi J,
First of all you've done well in keeping your subjects very comfortable and relaxed in front of the camera which is very important in shots like these so, it's just a case of getting your lighting balanced a little more.
By the looks of it, you have everything needed to get good shots so that's not a problem for you and sometimes it's not so easy finding your way round new equipment, especially when it's powerful as in this case.
Might I suggest you move your subjects further away from the background - this will allow the bounce-back from the white background to fall off a little and lessen the spill onto the subject - also try not to 'over-blow' the white background in order to get it fully white, which is quite a common mistake and makes the backdrop compete with the subject.
3/4 to 1 extra full stop over your key light is usually more than adequate to kill any key-light shadows and give fully white backgrounds if the lighting is even - if sufficient distance is kept from the back of the model to the background it wont bleed onto the edges of the subject, which also can be controlled by simple flagging if still needed.
It will also depend upon the size of your studio too - if it's small then alot of the bounceback will bounce around and find it's way onto the subject which throws everything out and flattens the lighting so, work to as low a light level as possible to avoid this.
For most studio shots of single subjects, try to get just sufficient light in order to set the camera to about f8 initially - most pro quality lenses perform best between f5.6 & f8 after which, IQ can begin to suffer. Obviously DOF will depend upon the focal length used but if one can use a longer FL then it's perhaps better to get the subject further away from the background and use a shorter FL.
I always try to build the shot from the back for a set - by getting the backdrop correctly and evenly lit first, it's then quite simple to light the subject with your key, fill and hair lights.
I notice you are using a cloth backdrop - a better material would be either a white Colorama paper or a good vinyl as it's often very awkward to get the floor area wrinkle/shadow free when it's a cloth backdrop. As long as the subject's shoes are clean and a decent ply base is used to support the paper or vinyl, it should last quite a while.
I'm quite sure that once you have got your lighting set-up tweaked, you'll be turning out great shots every time you use the gear.
Regards...
Tony