When I prep a model for 3d printing I usually have to wait for the web app at shapeways checks it for problems then fix any "thin wall" errors that are found. Sometimes I have to re upload and fix things several times in a row taking at least 20 minutes to upload and run the check each time. Sometimes I'll fix one problem and accidentally cause another.
3d printing can be fairly complex.
Note: Full color 3d printing has far more requirements than most other materials as it uses a fragile material that ink can be applied to the outside of.