In this day and age, businesses and customers are more likely to fail to pay you for money owed.
Clients need to have internal policies to monitor their accounts receivables regularly. When accounts are late, special letters need to go out after 45 days. More pressing letters need to go out at 60 days and 90 days.
You need to have trigger points to call an attorney and start a collection lawsuit. Do not sit on old accounts. As a general rule, the older the case, the harder to collect.