Custom style.css not updating

I am using a child theme on my website and updating the style.css file through ftp. All the edits were working just fine until yesterday.

I last updated the stylesheet yesterday but the changes are still not visible on my website. Nevertheless, the css turns out to be correctly updated both in the ftp and in the WordPress Editor page.

It seems like a cache issue, so I tried to:
– hard refresh the page
– clear the browser cache
– use other browsers
– deactivate all the plugins
– switch to another theme (which worked fine) and then back to my child theme (but still no change was visible)
– use Varnish HTTP Purge
– edit the css directly from the WordPress Editor page
but nothing seemed to work.

I know some providers need a bit longer to restore the cache and show the edits (some 20 mins) but it’s been more than 24 hours now since my last style.css upload and still nothing happens. Any suggestions?

I am running WordPress 4.3.1 with a DW WallPress child theme.