That is most certainly alarming to say the least. For now don't panic. What sort of commissions do the three of you have? Is it a full illustration is it a very complex work? And what about the completed artworks that came after yours? Are they simple or they are very detailed illustration? How long have you guys been waiting? And how is the average wait time for this artist commissioners?
I ask because I sometimes try to balance one or two very detailed artworks, one or two medium artworks, and then I'll sprinkle in some sketch commissions here and there. I juggle everything around so that I can keep interested in what I'm doing while also maintaining some income flow. Not having a clear and concise communication is definitely questionable though. I myself try to have a 3 to 5 business days reply back. If you just sent the message in the last 24 or 48 Hours, saw that the message had been opened, and did not hear back for now don't worry about it. Most businesses you do need to allow for a few days for them to write back in an appropriate manner. I know I sometimes open a message and will not reply back for 3 to 5 days. Because when I reply back to a client I want to make sure that I'm in a good State of Mind to be able to respond politely and professionally.
Meanwhile I would start to take screenshots of your payment and any correspondence you have had with said artist. Maybe give a phone call to PayPal to figure out exactly what can be done in case things do need to get escalated. Calling PayPal is not a big deal at all, especially if you're just looking to get some information. Hopefully you got all of this payment done by invoices and not as a gift.
Hopefully everything works out for you. If not then I really do hope that you're able to get your money back.