You’d be surprised at how many invoices don’t have specific due dates. Instead, the invoices might say “due upon receipt” or “due within ___ days.”
Why leave the math to your customers? The better practice is to include a clearly labeled “due date” with a date certain by which the payment must be made.
If you really want to get their attention, you could also include a date on which you’ll take the next, clearly defined step.
This may be giving the account debtor a deadline to pay of July 18, and, then, in the next time, telling them that, if that amount isn’t paid, then you’ve been instructed to file a lawsuit on July 19.
Granted, some debtors simply don’t have the money to pay, and no deadline is going to put that money in their account.
But, for those debtors who have the funds to possibly pay but don’t have your debt prioritized, a clear and unmistakable deadline–with a specific threat of the next action–could do the trick.