The small one looks to be crowded by other plants while the big one is not.

Plant experts can correct me if I am wrong but I think every individual plant has a specific genetic makeup that gives it some inherent characteristics just like humans. So, there are genetic predisposition to being tall, fat, scrawny, well built among humans (homo sapiens) of all kinds that cannot be attributed to environment/upbringing. I think this is likely true for plants as well. I guess an argument against this would be the presumption that named clematis would have plants that are all clones (vegetative propagation from a single plant via cuttings or mericlones?). Probably some are grown from seed? Anyhow, going by this theory, the only solution to a wimpy plant is to find a replacement that appears to have the makings of a robust plant.

There are Clematis that grow to 3' and some to 30' they do not all grow the same, some are much more vigorous than others.

buy or sell, these are exactly the same clems. When "wimpy' blooms the flowers are same as "robust' I checked the sunlight at different times during the day. Theres a small version of ROS growing nearby but wimpy clem got plenty of sun. The sun is hight= right

now. As it gets lower (after June20/21 many parts of garden get shadier. Not this one.

Im certain both get same amout of sun. im inclined to think of them is just less robust

any other ideas

No ideas Marie but your garden looks lovely!