This knowledgebase article details a know issue when using Neverfail Heartbeat.
When Neverfail Heartbeat is installed, the Secondary server (and Tertiary, if it exists) are created by cloning the Primary server. Therefore, all the local users from the Primary server will be cloned to the Secondary (and Tertiary).
If new local users are created on the Primary server after the installation of Neverfail Hearbeat, these will NOT be replicated to the other servers. New local users with the same name and password can be created on the Secondary (and Tertiary), however, these users will have different user ID than the ones on the Primary.
To make sure the users are the same on all servers, including the user ID, the Secondary (and Tertiary) should be recloned from the Primary server. In case of physical servers, the following Knowledgebase articles can be used:
Knowledgebase article 851 - How to Replace a Windows 2003 Server
Knowledgebase article 1774 - How to Replace a Windows 2008 Server
Knowledgebase article 2613 - How to Replace a Windows 2008 R2 Server