caching issue: doesn’t display new posts immediately

Just started using the plugin & am supremely happy with it. But a reader based in Israel says it sometimes takes as long as 3 hrs or more from publication of a new post to when it displays on the homepage. He can see the post by using the direct post link. But it’s not on the homepage.

I can see the new post on the Homepage immediately.

I told him to clear his cache & refresh his browser, but it didn’t help. Could there be a setting that needs changing to get this working properly?

My theme is Genesis.