In AEM 6.3, when activating a page, a page version is not created in spite of having all the configurations for the replication agent in place.
There are multiple possible reasons for this problem:
The Version Manager has createVersionOnActivation option disabled.
Go to http://aemhost:port/system/console/configMgr/com.day.cq.wcm.core.impl.VersionManagerImpl
Re-enable the createVersionOnActivation checkbox.
The replication agent has No Versioning option enabled on the Triggers tab.
Go to http://aemhost:port/etc/replication/agent.author.html
Edit each agent.
Select the Triggers tab in the agent's configuration dialog and uncheck No Versioning.
It could be that the system / service user permissions were overwritten by a package installation. The version manager uses a service user to access the page to be published. Before AEM 6.2, this was done with an admin session. On AEM 6.3 and later versions, access control entries (ACEs) exist for /content and /etc for the version-manager-service service user. These ACEs allow it to read and write to these paths (i.e. jcr:read, rep:write and jcr:versionManagement permissions).
See this article for details on restoring missing service user permissions.