Are you a bank? Why are you lending money to customers at no interest?
The global economy is heading for a downturn. So why are you not collecting from your customers that are over 120 days past due ?
You must be new to credit and collections or you have a leader that is telling you not to worry about it. These leaders will be gone in a year and you will be stuck holding the bag. Get your money now by turning up the heat. Send a Registered letter demanding payment or you will go to collections. Send an email with your collection partner on that email letting your deadbeat customer times up.