If you’re using the current public/stable version (1.9), it has not imported Web pages well for an embarrassingly long time. My understanding is that it’s because the underlying code that Scrivener uses is out of date for Websites as they are currently structured. It will be fixed in the new version (and may already be fixed in the current beta). Till then … saving the Web page as a PDF and importing that is about the only thing that works reliably.