Answer: The customer refuses to acknowledge you when you make contact.
Step-by-step explanation:
An early sign that a Customer will be Uncollectible is that they refuse to acknowledge you when you make contact with them. They will try to avoid you because they are unable to pay or don't want to pay the debt for whatever reason and as a result do not want to see or acknowledge you so as not to have to explain. Once this begins to happen, you must prepare for the possibility that the debt will be Uncollectible.
This is an early sign mostly because you are still in contact with the customer and there has been no reason to suggest that the debt will not be paid.