Some people's bodies aren't made for full time jobs, or their jobs are too hard on them. When I worked for Wal-mart, I would get home and immediately fall asleep, whether I tried to or not. :/ (And then I had to work again at 7AM.)

It sucks, but that's just how it is. In those cases, the artists shouldn't be taking commissions or should refund them (and at least the artist tried - though I would have just completely refunded them anyways).