I noticed that when I put an indent on the top (or bottom) of my model the software does not put skin on it. For example, as seen below, I put text on top of a model and indent it a full millimeter. When I slice it with Cura 3.2.1, there is no skin, and the infill can be seen through the text. When I slice it with Slic3r, I have four full layers of skin under the text, as it should be.