Here's the trouble I had with the chinese cobs, woodsroad. They were full of "leaks". The stems did not fit well with the cobs, allowing air to pass when I attempted to draw... the cobs themselves also allowed air to pass in and out through micro-holes in the sides and in the bottoms. All that "leakage" kept the tobacco from staying lit no matter how much one huffed and puffed.
Like I said earlier... three of them did turn out to be fairly decent smokers. The way I found out which ones they were was to hold my thumb over the top of the bowl, forming a seal of sorts. I then attempted to draw through the bit... If there was no airflow into my mouth, then I knew that they were ok. The bad ones let air flow easily even with the bowl sealed off.
Hope that helps.