Part of the answer to this was just covered. In general, you should consider all accounts that have not responded to your in-house collection procedures by 90 days as problem accounts. Take advantage of the 10-day free demand period, or Final Notice, as it is called. This gives you the benefit of third party collection leverage, without paying a collection fee, for 10 days. If you don't get paid in the 10 day free demand period, your ex-customers should be pressured by a third party collector.
Timing is critical.
The difference must be written off due to the fact that if a company delays third party action, their colleagues in the industry will not. When the company finally acts, any remaining cash will be gone. Probably the bank will be there ahead of you. Notice there is no suggestion that third party collectors are superior to any other collectors. They are not, but they do have tremendous advantages.