CD, could it possibly be the needle is too big for the thread? When I got the 31-15, the only needles were a couple of 16's and it skipped stitches all over the place.
When my order for needles came in, I put a 12 in it and it worked perfectly. The only thing I can guess is that with these honkin' big industrial needles, the hole punched into the fabric is too big to put enough friction on the thread as the needle ascends. No friction = no loop for the hook to catch, right? I hope you can try a small needle (I think the smallest DBX1 I could find was a 12) and fix the problem.
I have question for you, though. How did you set up the motor? I want to put a small household motor on the 31, but there's no place to mount one. I'd love to see how you got that to work! :)