If a piece is not finished at a show, I ask for a solid completion date. An artist was to complete a piece for my for NYCC but it wasn't done. I have been told it will be done by the end of the year.
If it is not, I will ask for a refund. It was facilitated via the artist's dealer so I don't expect to have any issues.
Does the artist you are having an issue with have a dealer who sells his art? I would try contacting that person. If not, and you are fed up, I would contact the show promoter where you saw the artist and ask if they can help. Obviously stating the artists name may result in him contacting you.
Many people no longer take money down, or only take a partial amount as a deposit. This is how I prefer to have commissions done.