The instructions suggest "If you have a hard clog that prevents you from extruding at all, you will have to dislodge it before proceeding. The best way to do this is to find a thin steel or brass wire (electrical wires like aluminum and copper are usually too soft) that will fit up through the nozzle (bristles pulled from small wire brushes work well, thin music wire works too)."
I was never able to find a bristle from a wire brush that worked for me. The bristles that I found were either too thin, and bent very easily, or too thick to fit.
I finally discovered acupuncture needles. 28 gauge (0.35 mm) is the perfect size. What worked for me was to grab this near the pointed end with a needle nose pliers, and slowly push this through from the outside of the nozzle to the inside of the nozzle, while turning the needle. If you push too hard, the needle will bend, but if you're patient and push gently while turning, the pointed end of the needle should eventually break through the hard clog.
Note that in the USA, "Federal law restricts this device to sale by or on the order of qualified practitioners of acupuncture." However, I found a local supplier that was willing to sell me a box of 100 without checking my ID. It is likely that there are online sellers that will also sell these without checking ID.