Let us for example say your installation default for BKRETPD is the default 14 days. You have a job a backup with a 67 day retention. Your backup datasets seem to be disappearing after a weekly purge about two and a half weeks after the backup is recorded. Why aren't the backups kept in the JCSCDS for 67 days?
Keep in mind that CA FileSave doesn't care about RETPD/EXPDT in JCLs which create new backup files, but only when you are using those parameters together with CA FileSave commands.
In our example it is reading the expiration date from DSCB of the dataset.
You can always try to change BKRETPD in RCSYSGEN macro (CA FileSave Installation Guide, Detailed Installation Steps - Step 16. Define the System Generation Parameters) instead of using parameters in JCL.
But if there are some storage limits set up for expiration on the system that cannot be overridden then parameters that exceed the limit will be ignored.
It is suggested that contact your storage group and to check your retention period settings to make sure they don't override BKRETPD and delete backups before you want them to be.