Hi, just an update: I can't believe it took someone to point this out, but the easiest way to test if the problem was internal or external would be to use the machine's own designs. Sure enough, unfilled outlines and trouble with satin stitches on a very simple internal design.
I've contacted the seller (I bought it online from Ken's Sewing), since I'm pretty sure it's a machine issue, not a user issue.
Thanks so much for your responses.