It's now 12 ft tall but with only few thin stems/trunk. I planted it by my honeylocust tree. Is it because it's not getting enough sun?
Possibly but the fact that it is so thin and scraggly is a bigger reason. You should have pinched it back to make more stems. More stems equals more flowers. They bloom on the tips on all along the stems.
Do they bloom on new stems? These are all new stems.
I'd say not enough sun. I had a similar experience with several of my clematis. Ramona really likes her sun.
This one gets full sun all day, I never fertilize or water - it's mulched heavily and never disappoints.