Ola Hallengren solution, backups not cleaning up
This is a list of random things that come to mind, I can't guarantee any are the problem.


DateTime on Server has been knocked out
DateTime on the Backup store has been knocked out
Someone has changed the @CleanupTime on your agent job (potentially accidentally)
There is a backup of your backup location occurring while the cleanup is trying to run locking the files (have had this one myself)


Since you've said that this is a recent problem not a just happened I'm presuming you can still delete the files manually which would indicate they are not in permanent use.

It may be worth running the -o command at the end to write the output of the command to a file ( where it is '-b' chance to '-b -o C:\Logs\backuplog.txt') for example and consult that for if it is bringing back any problems