Give it time. I don't think the black surface is the problem. Younger plants may need help in grasping onto surfaces but as they mature and their vines get larger they can easily grasp onto much larger items.

I find most of my Clems need winding or tying, they don't cling on their own well for me at all.

That is really strange BorS. The exact opposite experience from what I see in my garden. Wonder if the type of clematis is the difference since I tend to grow more type IIIs and I believe you are more into the type IIs?

Does the black trellis get really hot to the touch? With our thin Colorado air, the sun is so very strong here. But, that said, I have some clematis on dark metal trellis that are doing fine and curling around the bars, but most are on the east side of things, which keeps them out of our crazy Colorado drying winds.
Twenty years ago I planted my Jackmanii up against a fan shaped wooden trellis that it's never wanted to cling to. I've been experimenting with different things to get my Jackmanii to cling and this year it looks like I succeeded with plastic vegetable netting that has 4 inch holes attached to the wooden trellis. It's doing a very good job of curling around the netting.
Unfortunately the netting is white, but very soon you won't be able to see any of it.
The Jackmanii is on the east side of the house and only gets a half day of sun, but it blooms like crazy every year.
I have a pic in the camera if you're interested.

Thanks for the input everyone... all my other clematis are clinging well to chicken wire, vinyl coated fence, netting or shrubs. This is the only one with a problem. I think I'll try attaching netting to the trellis. Although fastening may be a bear! The other place I used netting was with a wood lattice.

Thanks again!

You may need the netting only on the bottom 2 or 3 feet. I have plastic thin bird netting on some of my "structures" and others are bare black, brown or green metal. I had to do a lot of tying to get them started, but once they get about 2 feet long the tendency to grab really kicks in. I used some canes for ones that were further away from their support, and by the time they got to the trellis I had to detangle a lot of grabbing leaf stems from the canes.

And some cultivars to seem to be more grabby than others.

Black metal trellisesor pots have never been a problem for me, but I can only garden on the North & East sides of the house due to the neighbors privacy fence and 2 ancient Maples on the West side & a sharp dropoff to the South. And that's my dog Rusty's domain when he is out on a pulley rope from the back door to a tree on the bottom flat surface, near a wooded area.

Marie