Unless you have the luxury of an in-house credit controller - which is something even some larger firms can't afford - you might be tempted to take a head-in-the-sand approach to chasing overdue invoices, and simply try to pretend they never happen.
Sadly they do happen, even from trusted long-term customers, and that can lead in turn to some soul-searching: Why didn't they pay? Did I do something wrong? Is there no trust in business any more?