Is it your opinion that the casings were tough? If so, the casings were not soaked long enough, or the links were under stuffed. Just throwing that out there as a possibility. You say you keep the casings stored in a air tight container. Did you freeze the casings? If so, that could be the problem for casings should not be frozen. Were the casings stored in salt in that container [which is the proper way to store them. I'm assuming that it was the casings as well being that you used the same meat mix and procedure in the cooking process, Cooking the sticks at too hot of a temp and too quickly is another reason for toughness but like you said, you cooked them the same way as always. How old are your casings and were they stored in salt in the fridge all this time? Reinhard