Css not updating

HTML & CSS
1

I make changes to a css file, upload it to the web server and reload the php page which links to it but a previous css file version is being used.

I have rebooted to clear any cache but the same thing happens.

I change the php file and get instant effect when uploaded.

Any ideas anybody?

Thanks

2

I create a duplicate css file with a different name and link to it in the php file
and it works fine.

I link back to the original css file and the changes are not implemented.

Even more weird.

3

Sometimes if you do a second page refresh the new version of the style sheet kicks in.

Another way around it is with a timestamp:
Can We Prevent CSS Caching? | CSS-Tricks