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Hey MV - What you say here I don't think is the correct way to be thinking about it: "My thought would have been that the sub domain is only as good as the main domain and in this case the main domain isn't strong because little or no content is hosted on it." As I said in my original comment, there isn't really a "main domain" with a technical implementation. Your subdomain should be able to rank just fine as long as it is the canonical, has links to it, and the migration was done correctly from the old to the new. I still suspect that there are a lot of links left to the old site setup that are not yet benefitting where your site is now. A subdomain is a separate site from another subdomain, so site.domain.com is different from www.domain.com and has its own ranking potential. Links to www.domain.com may have a knockon effect for ranking site.domain.com, but you also need links to site.domain.com to really be able to rank.

As Logan pointed out, the most likely cause of this is because of the canonical tags pointing to the HTTP variation. I'd probably suggest carrying out the following: Download a tool like Screaming Frog SEO Spider and crawl your full website - https://www.screamingfrog.co.uk/seo-spider - Once the crawl has run, export the data and check both the URLs and the Canonicals across all pages to ensure only HTTPS variants appear. If you find http variants, then it's likely that the redirect rules you have in place aren't working. This is a global rewrite rule that can be applied to force the https:// version to show at all times, so you shouldn't need to make the changes once by one. I'd also make sure that if you're linking to any pages internally, that you're linking to them using a relative path, or straight to the https:// version. If your canonicals are causing issues, just ensure they're always choosing the HTTPS version and you shouldn't have any issues! Finally, in GSC, just make sure you've got the HTTPS version of the site setup and that your sitemap includes only HTTPS URLs. Hope this helps!

Hi, That's a great help, thanks a mill! Leaving them in place as a fall back makes sense.