SQL Maintenance Cleanup Task 'Success' But not deleting files

I have a maintenance plan setup for a databases on a server. As part of the backup is a Maintenance Cleanup Task.

SQL Version 2008

The task that 'succeeds' is setup as:

Delete backup files
Correct folder (same address as the backup task)
File extension: bak (NOT .bak)
Delete files older than: 20 Hour(s)

I have other similar cleanup tasks that occur in the same maintenance plan which work fine. This plan has worked fine in the past, I just noticed that last night it reported 'success' and the rest of the plan continued, however the file from 2 days ago still remains.

I have checked similar questions such as this question, and this is not the case as my maintenance task worked fine two days ago and for the past several weeks:

Check the file to see if it is a valid backup file. If the file is corrupt or in some other way damaged the process to remove the file won't do anything. You can check it by doing:

RESTORE HEADERONLY FROM DISK='d:\Path\To\Your\File.bak'