OK - it never feels that simple, you are a human, and you're dealing with other humans (most of the time). And there's an argument to say if we were all a little more human in our interactions as businesses - things would be much better.
If the client who isn't paying you on time is a faceless megalithic corporation which passes you from department to department, you're fighting a battle with an organisation that's unlikely to feel the pressure of you nagging.
If the client who isn't paying you on time is a small business themselves, you might feel guilt putting pressure upon them paying on time, and reasons like "our client has paid us yet" are easy to understand and feel sorry for, especially under challenging circumstances like post COVID.
Indeed, the number of B2B invoices that are paid late has risen during COVID by 53%, and 29% have had their payments frozen altogether (FSB).
But the problem goes all the way down to the bottom - for every invoice which is paid late, there is a knock-on effect to a dozen other businesses and individuals.
That's why we, as businesses, need to be 100% clear on saying no to late payment.
We need to collectively work together to not accept late payments, else the problem will never go away, so even if you're "okay" with getting paid late, make a stand to support others.