sorry if this is coming too late - I had a look at two of your models. I see that hey are not manifold. If I check them in Meshlab I see small red dots on the Complex Weave on the place where ribs get into the main, I don't know how to explain it better, I wish I could upload a pic to show it. The preview generator cannot generate the picture because the model show errors. same happens to a lot of other models, not only yours. You have to clean the design before uploading it if you want to get a proper preview picture or you can take a screenshot yourself and upload it as you did already.