Err, I think you have it backwards? You want the thicker rope through the rap rings. Anyway, that's how we ended up doing it after getting it stuck, and it worked for another 8 or 9 raps or so. I don't think it's so much due to rope stretch, but due to the ropes being different diameters and passing through your device at different rates -- i.e. the rope didn't "Stretch tight" -- it actually slid and moved until the knot got caught.
When we placed the knot on the other side of the rings (i.e. pull skinny), the knot continued to move, but it moved away from the rap rings, instead of into them.