Does anyone know why readers don’t work for some websites where it looks like they should - ie normal article with lots of text.

You just get a completely white page (on the iPhone reader). Usually it’s a news website.

Is this the website intentionally obscuring the content to ensure they can serve their ads? If so how do they go about it?

Cookie and "we care about your privacy" banners are often the cause here, especially if you're in the EU / UK / possibly California[1].

On some websites, those are just modals that obscure the content, something that reader mode can usually deal with just fine, but on others, they're implemented as redirects or rendered server-side.

If reader mode doesn't work, dismiss those first and try again.