I fully understand your frustration in observing the lack of success in recovering former tenant's debts. However, the reality of the situation is that they are notoriously difficult recover economically for some of the following reasons,
- The landlord may not know a new address.
- The landlord has very few punitive measures which can be taken against a debtor, other than adding more debt to the total (which tends not to scare the debtor)
- It is possible to obtain an "attachment to earnings" on the debtor (Court Order deducted direct from their salary), but they are often not in work.
- The cost of chasing the debtor, either through a contractor or the direct employment of specialist staff, is often prohibitive.
Hope this helps to show how difficult the problem actually is.