Marc,
I stopped by your blog and viewed every photo. On the first two photos, the model-in-the-city type feel in the first B&W photo is kind of spoiled by the distracting background elements. When shooting in B&W, design elements like line,shape,and mass take on huge importance, and the strong lines and large sizes (high mass) of the telephone poles and trees behind the subject are really,really competing for attention with her. Those black shapes behind her really dilute the impact of "her". Her foot position also looks awkward,and unnatural in the B&W shot.
The second shot of her in the city has a much more "tame" background, but there is the fork of a tree and a street lamp right behind her head,and those distracting elements are competing for attention with her face. I think the background is very important in these types of urban photos---the whole idea is woman in the city, so the background has to support the concept, and not take away from the "woman" who is in the foreground. One option is to use a longer focal length lens from farther back, which will give a narrower angle of view,and more background blur in the out of focus areas. The other option is to be really,really careful of the background, and to frame the shots very carefully.
I know you have a 45/2.8, which shows a reasonably wide angle of view when used to capture full-length figures on 1.5x, so if you want to use that particular lens, you need to look first at the background and then bring the model into the angle of view the lens will see. You actually have to compose "backwards" when using a wide field of view lens when doing location shots. Find the location, see how the lens renders it, and then position the model.