I've had a great deal of difficulty maintaining contact with the following two customers after completion of preliminary sketches:
Kraken D'waggin (only known alias)
Thom Wolf (Thomas Schwarz)
Initially, there were reasonable excuses for the lack of payment - Paypal difficulties and ill health respectively - and I will always take an excuse at face value if it seems reasonable.
Since then, numerous attempts to contact these two customers have been made with responses tailing off and then simply ceasing. I would advise that any other artist attempting to work with them secure payment up front since this is the only issue with these two customers.
Both were polite and described what they wanted perfectly, the transaction has only broken down at the payment stage, which is regrettable.