Define "several". I know that 15 is the max, and so I think there's perhaps only a 10% chance that you get the max... It's been a few months since I was making HCI 5% studded stuff with spined kits, but I seem to remember needing about 1 kit per piece, which is ~10% success rate.
So, if you've gotten, say 15 pieces of eater, but none were 15%... well maybe you're just a bit unlucky, but not enough to indicate that it's not possible.