Varnish is not caching my site.
Two common reasons for varnish not caching the site are -
1.) The max-age value has been set to 0, under the Performance section on the site at - <example-site-name>/admin/config/development/performance. If that is the case, varnish will not cache your pages, respecting the max-age setting.
2.) Your sites are under development, and you have shield in place. Having a basic authentication/shield in place will prevent varnish from caching the site.
3.) You have a "no-cache" header for your site. This can be unintentionally set via an uncached View, for example.
If none of the above two use cases fit your issue, please file a Support ticket with Acquia Support.