Two common reasons for varnish not caching the site are -
max-age value has been set to 0, under the Performance section at
/admin/config/development/performance.If that is the case, varnish will not cache your pages, respecting the
- 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.
If none of the above two use cases fit your issue, please file a Support ticket with Acquia Support.