This article explains why the audit trail is not storing comments from requisition approvers
If an approver of the requisition adds comments before sending it to the next approver. Then the option "Do not store sender's comment on workflow action" is enabled.
Disable “Do not store sender's comment on workflow action”.
Admin Tools > Company System and Logo Settings
By default, the SuccessFactors Application stores the comments in the system database, and displays the comments in the audit trail.
When you select "Do not store sender's comment on workflow action", the SuccessFactors Application will no longer store these comments in the database, and no comments will display on the Audit trail. With this feature enabled the comments column in your audit trail will be blank, even though user is writing comments prior to routing the requisition.
KBA , LOD-SF-RCM , Recruiting Management , How To