i have never seen the machine or accessory you're talking about, but i have a guess about the problem.
i'll bet the problem isn't you or the machinery. i'll bet your real quilt is just a lot larger and heavier than the samples you worked on. AND i'll bet that with practice you will love your real quilt results as much as you did the samples. :wink:
have you considered buying a couple of cheap sheets and batting so you can use them to practice working on big items? i am 100% sure you'll get the hang of it before long.