I recently had a similar problem and found a solution.
I use DreamHost to host my site so i’m not sure if it will be the same for you.
My style.css file was also not updating. I cleared my cache on my browsers and re-loaded the style.css file with no luck. Finally I found out that DreamHost uses this thing called “Varnish” to cache files like my style.css.
After downloading this plug-in: Varnish HTTP Purge
and clicking “Purge Varnish” on my WordPress Dashboard, my changes showed up!
