Https not being used by w3C

I have a strange issue which has actually come up in a couple of sites using W3TC where the cache isn’t using using https.

In the most recent site, it was originally built in http and then changed to https. That was months ago. On my browsers the site is working. But on my clients computer she the site isn’t loading for her because assets are getting blocked that aren’t using https (eg the minified cache files, theme images etc).

What might be causing this? Is there setting for telling the browser that it needs to refresh it’s cached files for the site?

Thanks.