Not quite positive what your experience is with the slug not locking, but make sure to vaccuum the hole once in a while. On the first few balls I did, I noticed there is a little bit of dust or grains of ball filler that kinda end up in the IT at the bottom of the hole. This was preventing one of my balls from "clicking" into the lock position. It would kinda just "slide" into the lock position. Vacuuming the hole remedied this problem. If you are unable to even get the slug to screw in, it's possible the IT is not centered in the bottom of the hole. This can happen if a lot of vibration occurs and the table shifts slightly before drilling the 1/2" hole. And then once the sleeve is installed, the slug may not line up with the IT, therefore preventing it from even getting into the threading.