I had the same problem with mine. I planted mine last year and it started growing fine this spring but stopped dead in it's tracks now at only 2 ft tall. It's totally stunted :(
Either mine is resting and growing roots or I've got a problem!
I may have to do a soil test of that area and see what it needs. It should be doing "something"! I may try some alfalfa tea on it.

I just planted my first one this year. I'll throw this out there - maybe those more knowledgable can chime in, but I remember reading that Sweet Autumn Clematis is a common name for various genus or species - whatever the proper terminology is - and that you have to be careful who or where you purchase it from. ??????? I'm going solely off of memory... I wonder if this has something to do with it. I'm curious myself.

Also, with my other clematis, mostly type 3's, I prune back the vines so that they start branching out at the height I want them to, and also because this helps to establish root development the first two years, thus creating a bushier plant. Whether this is recommended for the SAC, perhaps the experts can chime in on this as well.

Good luck. My vine is now approx. 4 feet tall, but not much branching, just planted earlier this year.

I got mine at a very reputable source, Garden Crossings. I did prune mine this spring down to the 6 inches or so for the group 3 vine. I can only hope it's growing roots! I may have to move it if it does the same next year.

I remember that happening to mine, so I kept pruning it to about 2-3 feet. The third year it finally looked like something, and the 4th year it was a monster.

You may choose to check your state's listings of invasive plants to ensure that C. terniflora is not on the list. Seems to be more of a problem in southern states. Sweet autumn clematis is largely considered an invasive plant here in the Philadelphia, PA area.

Here is a link that might be useful: Sweet Autumn Clematis