For problems saving PDFs from ScienceDirect, see this forum post.
When you're unable to save high-quality metadata from a particular website, most likely either the page isn't supported by an existing Zotero translator or the page layout recently changed, breaking Zotero's ability to recognize data. On some sites, most notably Google Scholar, you may also be running into site access limits.
If the problem occurs across multiple sites, there may be a problem with your installation, and you should try these steps: