It makes no sense to me when an artist drops the ball on an almost-complete picture, yet works on other things.

The only thing I could suggest would be to send an email to the artist's email address (not an FA note), asking them to please give you an estimate of when they will finish it. Perhaps suggest a deadline (say, a month from when you send it). You could try sending it to the same email address as their PayPal, if you don't have any other email for them.