According to a recent small-scale analysis, more than 80 percent of HTTPS URLs are not showing up in Google's search results, said Gary Illyes, a webmaster trends analysts at Google, in a post on Google+ this morning.
Despite being eligible for indexing - many of the HTTPS URLs don't have any crawl issues and don't have a noindex - these links are being displayed as HTTP URLs because of webmaster configuration.
"[The URLs] can't become canonical because website owners don't tell us about them. They use the HTTP variant in their sitemap files, in the rel-canonical and rel-alternate-hreflang elements, even though the HTTPs version works mighty fine," Illyes wrote.
If a site supports HTTPS - to which Google gave a ranking benefit back in August - Illyes recommends using them.
Have you told Google about your HTTPS URLs?