Check the status of those computers. Can it be that they are not "active" anymore or actually not managed by this NS?
Computers to purge are determined by a stored procedure called dbo.spGetComputersToPurge and that will only purge computers where the "ismanaged" column returns 1.
Automatically deleting resources which status is not "active" is rather challanging as there is no task to actually delete resources.
In the past I've been using either workflow solution to do the deletion or built up an "internal" connector with connector solution which merges all computers based on status "retired" into one resource and then manually deleted that resource (saves a lot of clicking ;) ).