Changes made to (static) CSS file not reflecting in Django development server

In my case, my browser was caching the Js and CSS files and thus I was unable to see the changes.
You can try hard refreshing the page
For chrome/chromium browsers:
Ctrl + F5 or Ctrl + Shift + R
both works for me.

If you develop locally using Django's server there's no need to use the collectstatic command, it is only meant to collect all static dependencies to a single access point where you can point your 'real' server to, e.g. apache, lighttpd, nginx etc.

As for your problem, you insinuate it 'magically' fixes it self after a few days. Have you tried resetting your browser's cache?