I agree--urethane-coated nylon deteriorates relatively quickly. I suspect it's gumming up your needle. IMHO, using cheap (or free) fabric usually comes back to bite you. When you consider the blood, toil, tears, and sweat that go into a DIY project, it's worth using fabric that will last, not fabric that's undoubtedly on the verge of deteriorating if it hasn't already.

I also recommend making up a "rough draft" of the pack in a cheap material (cheap muslin or, as Pika suggested, old sheet). That way you know it's what you want and, especially, that it fits. Then go out and buy some good stuff, like Dyneema, that will last long enough to make the effort you're spending on the project worth while. I've been through this with clothing.

While at Walmart, look for a small remnant of nylon ripstop (I've been told they carry it) to practice. If your machine works on that (with a new needle), you'll definitely know it's the old urethane coating.

As I stated in response to your previous post, don't forget to change your sewing machine needle frequently, like every 4-5 hours, when sewing nylon! Nylon, polyester and other silk-like fabrics (as well as silk) dull a needle really fast!

Good luck!