Selection only showing part of the object selected

I'm shooting sports poses on a white background. The problem I'm having is when I select the athlete their sneakers are fully selected but when I add a solid white background the bottoms of the sneakers (white) are not fully displayed. I start with a levels layer and darken the subject to get fairly clear contrast before I select the subject and make the adjustments needed to make sure I've got it clearly selected. These are full body shots so it's critical that I resolve this as soon as possible hopefully with the help of this community.   I've searched online and tried the couple of different methods they suggested without any success.  I'm on a mac, using the latest Lightroom build and Photoshop Beta 25.1 release. Thank you for any assistance you can provide.