For the thicken feature to work, the curvature of your surface has to be consistent enough that the feature will work.
The thicken is failing at the first boundary surface.
You can see the curvature changes sharply at the back of the shoe. Typically the thicken feature will fail for cases like this. The back of the show needs to be made with smoother transitions and you should have no issues.
Another indication is that the curvature combs for direction 1, in the boundary surface, go from a small magnitude to a very large magnitude. This is indicative of a rapid change in the curvature and is one of the reasons that the thicken feature is failing.
To put it simply, you need to make the surface smooth and remove out any unnecessary inflections.