Hello Pankaj,
most common case:
customer coding which is directly updating usr02, or direct updates of the table with db-tools.
This bypasses then the changedoc creation.
solution: in your coding, onyl use bapi_user_lock (_unlock).
Further possible scenarios:
ush02-entries have been deleted manually
ush02- entries have been archived
user-IDs have been imported/overwritten by transport/ client copy. (many admins are not aware of the fact, that wich CC-profile sap_user the user changedocs are not copied. So after user import, the history of the users is showing 'corrupt' entries, logged chagnes are not representing in the acutal user, etc.
b.rgds, Bernhard