If a customer seems to have disappeared off the face of the planet. Debt collectors are trained in tracking down non-paying clients and have the tools available to help find people when you have lost track of them.
If the debt is over 90 days past-due, a debt 90+ days past-due is not easy to collect on, a debt collection agency has a much higher chance of successfully collecting the debt then you will.
Customers that have sent in a check or multiple checks that have bounced is usually a strong indicator that the customer has no intention of paying that debt.
When the debtor never calls back it should be taken as a clear indication that they are avoiding communication with you and will continue to stall your efforts to obtain payment.
If the debtor repeatedly breaks promises to pay, this is just another stalling tactic, and as the debt becomes older it becomes more difficult to collect on.