A canonical tag is a hint, not an instruction

What the tag actually does

A rel="canonical" link element tells Google which URL you would like treated as the main version of a set of duplicates. Google describes it as a strong signal — not as a directive it is obliged to follow.

“A strong signal that the specified URL should become canonical.”

— Google Search Central, "How to specify a canonical URL"

The other signals you are sending

Your declared canonical is weighed against several other things, and some of them are stronger than the tag. Google lists, among others:

• HTTPS over HTTP — the secure version of an otherwise identical page is preferred.
• Permanent redirects — described as a strong signal that the target should be the canonical. A 301 pointing away from the page you declared as canonical is a louder signal than the declaration.
• Presence in your sitemap — described as a weak signal, but a signal nonetheless.
• hreflang clusters, for sites with regional or language variants.

The mistakes Google names

Rather than guessing at best practice, it is worth knowing the specific errors Google calls out in its own documentation:

• Using robots.txt to try to control canonicalisation.
• Using the URL removal tool for canonicalisation.
• Specifying different canonical URLs for the same page using different methods — for example a link element that disagrees with a sitemap entry.
• Declaring a URL fragment (the part after #) as canonical.
• Using noindex when what you meant was rel="canonical". These do very different things: one removes a page, the other consolidates duplicates.

The practical consequence

If Google has chosen a different canonical from the one you declared, that is not a bug to be argued with — it is a signal that your other signals disagree with your tag. The useful question is not "why is Google ignoring my canonical" but "what else on this site is pointing somewhere different".

It is also worth saying that a site with no canonical tags at all is not automatically in trouble. Google is explicit that specifying a preference is not required and that it will pick a version on its own. Missing canonicals matter most where genuine duplicates exist and the choice is ambiguous.