Who are you dealing with?
Do you really know the legal entity behind the customers who owe you money?
You may be invoicing John Brown or White or Green but in reality you are actually dealing with a limited liability company. As a result the risk level of the debt could be a lot greater than previously thought.
For instance, they could be or have been a director of a related company which has recently ceased trading or a liquidator has been appointed. Once again, this has the effect of increasing the risk level.