I'm having a bit of trouble myself poking at a few artists who've owed me around similar times, if it means a lot getting your work from them then try setting up a deadline like many will suggest. Possibly widen the times between asking for updates, and whenever a new commission post is made poke them again asking if they were going to tend to commissions before the newer ones.

People are going to make excuses when they don't wanna do something, it's a fact and though it's unprofessional for someone with a business to do so they're still human. Just don't try and be a bother in the sense of sending a note daily asking the same thing and stalking their profile ya know? But you need to stay persistent with them if you want the art (or refund) though if they've done most I'd wait it out, but if they haven't even started ask for an update (sketch possibly) or they refund you simple as that.