The trouble is that you need to get paid, who can af ford to provide free supplies? So what do you do? First you try and reason with the debtor, sometimes this helps to solve the problem, but what if the the debtor still refuses to accept reason?
To do so he needs the excuse, which means that it is now your fault - not up to standard, late supply, the service did not provide the expected results and God knows what else.
Unfortunately it does not stop there. Now he has to keep face with his friends or business acquaintances in case they think he cannot afford the service and that ha has not honoured his commitments, so he now paints you in black and it follows that people you do not even know also start spreading bad vibes about you and your business.
It is very true what they say about the Costa: "Never beleive anything you hear and only believe half of what you see".
The next time your best friend tells you something negative about a supplier (or anybody for that matter), don't beleive it right away, ask the supplier. It may well be the case that he is owed some money by the person that bad mouthed him..