I noticed that when Shapeways rejects a model from upload, there's no error or indication thereof. If a model uploads successfully it goes through a bunch of tests and you get a "success" or "problem" email regarding printability, but I've now experienced multiple occasions where uploading a file in the first place fails and all that happens is the dialog box disappears. It would be nice if there was at least a "Something went wrong. Please check your file" dialog, if not a more specific one telling users that their file is too large or contains too many polygons.
More often than not.. such a problem is during the upload itself. There are a very limited set of cases where you won't be sent to the Model Edit page. It could be that either your connection cut out, or the file itself is >64mb