Pippi, I haven't had germination with clematis seeds, yet. I thought I did once, but it turned out that it was a cleome.
Some say to stir up the dirt (and seeds) in your pot from time to time...I'm not sure why, however; most just say plant...and wait. Three years for germination sometimes....
Lin has grown at least one from seed; maybe she knows a special trick or two.
The thread "Question re: Clematis cultivar Niobe" in
Clematis forum