Frontend not updating despite CMS working and updating content files (potential server issue?)

A client contacted me saying they could not update their website via the CMS. I’ve looked into it and it turns out they’re correct: changes can be made and saved in the CMS but the frontend of the site is not updated. Even more strange, I can download the content files from the FTP and see that the changes have indeed saved correctly. My local version of the site is also working fine, so I’m guessing it’s a server issue. Any ideas on what I can tell them, or how to fix this issue?

Might be a server caching issue, check if something like Varnish cache is enabled.

I assume they are making changes via the Panel?