I hope that you don't do that. Maybe your local butcher can let you buy a few yards of casing off him - then you'll know whether it's the casings.
If there's still a problem, try the butcher's casings with one of the tried and tested recipes on here. Use the techniques described in the Beginners guide (see the FAQ).
If it still doesn't work, then it's down to the cookiing.
Phil