Nodes not purging on update

We've been having the issue where purging of nodes after they are updated is very random and erratic - sometimes the node gets purged and sometimes it doesn't.

I checked the logs and I see entries for successful purges every time I update a node, but only sometimes does the node actually get purged and the updated content is shown to an anonymous user.

There doesn't seem to be any rhyme or reason to which nodes get purged and which ones don't.

My process for testing:

1. Update the body field of a node. The updated body content is shown to the logged in administrator user.

2. Visit the node in a private/incognito browser. For some nodes, the updated body content is shown to the user and for other nodes, it is not.

The only correlation I can find is that our more heavily viewed pages seem to not purge as often as our less viewed pages. This has me wondering if maybe these pages are actually cached by varnish and Acquia Purge isn't working at all, it just appears to be working on pages that aren't actually in the varnish cache and therefore don't need to be purged.

Just thought I'd throw this out there in case anyone is having the same problem and can help triangulate the root of the issue.