You might try a vacuum cleaner, sucking or blowing function (haw-haw), in both directions alternately, and see if that will dislodge whatever it is. From the diagram on this kind of chambered drilling, it looks a little tricky, and I'm really surprised it works as well as it usually does in Petersons or Dr. Grabows. Also a little weird is that the blockage is so solidly in place. I wonder if it could be something that melted and solidified. Anyway, good luck.