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.
Varnish is not caching my site | Acquia Product Documentation
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