It's growing underground, not above ground.
Clematis blooms on new growth only.
It takes a few years for clematis to really become established, at least 3 years.
Don't worry, it will take off this year again for you.
You have to be patient with them for 3 whole summers.
I use Miracle Grow on mine once a month in the summer, mabey that will help with new growth.
Good Luck!

I agree that the plants are probably just establishing their root systems. However clematis armandii is a type I clematis that blooms only on old wood, not new wood. That explains why the one that grew the most last year had the most blooms this year (more old wood=more blooms, not new wood). Patience is the key for most plants and clematis are no exception.