Varnish is not caching my site

Issue

Varnish is not caching my site. 

Resolution

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.

• In Drupal 8, this is under the Page cache maximum age section.
• For Drupal 7, it is under Expiration of cached pages.

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.