Posted: 19/09/2016 at 20:25
andrea, you need to know what varieties you have as clematis are divided into 3 groups depending on when they flower. Didn't they have any pruning instructions on the labels? You say they are starting to lose their flowers now. This could mean they are late flowering varieties known as group 3. If so, you would prune them in the spring. This could be down to a couple of feet as they flower on current years growth.
If you tell us what varieties they are, I'm sure there are people here who can give you more detailed instructions.