I didn't open your files (I'm still on 2010 and assume that yours are a later version). I am guessing that you may have too many small faces and when you try to thicken by the amount you want that some of them would disappear. In that case, I would guess that the best approach would be to try and replace the small faces with larger ones.
Another possibility is that you have areas whose radius of curvature is less than the wall thickness you are trying to create. You can use the Curvature display to see the areas that are giving you problems.
For both problems you can also try smaller offsets to help understand where the limits are.