expire_snapshot procedure in iceberg not expiring and not deleting files referred by older snapshot

Question

I am performing a POC on iceberg and trying to expire snapshots using below command.

spark.sql("CALL demo.system.expire_snapshots(table => 'db_name.table_name', retain_last => 5)").show(truncate=False)

however this is neither expiring any snapshots nor deleting any older data files. Output of above command as below.

+------------------------+-----------------------------------+-----------------------------------+----------------------------+----------------------------+------------------------------+
|deleted_data_files_count|deleted_position_delete_files_count|deleted_equality_delete_files_count|deleted_manifest_files_count|deleted_manifest_lists_count|deleted_statistics_files_count|
+------------------------+-----------------------------------+-----------------------------------+----------------------------+----------------------------+------------------------------+
|0                       |0                                  |0                                  |0                           |0                           |0                             |
+------------------------+-----------------------------------+-----------------------------------+----------------------------+----------------------------+------------------------------+

any suggestion what am I doing wrong here?

Thanks in advance.