You aren't doing anything wrong. I live in NW Florida, where it's typically warm year around, and mine don't reach 8' most of the time.

If you want a really good, tall annual vine, plant morning glories. My moonflower vine only grew about 5' last year. It didn't die back, so hopefully it will grow taller this spring.

Ok. What about a perennial vine that will go back next season even if it's planted on a huge pot?

Sweet Autumn Clematis is a perennial vine, hardy in your zone, that blooms in the fall and gets very large. Do a Google.com search for perennial zone 5 vines - maybe you'll find one that your like!

I've had purple hyacinth bean vines grow much larger than that, but they were planted in the ground. I ordered the seeds from Park Seeds. But then I have also bought seeds (can't remember brand) at the store that grew a lot smaller.