I have loads of clematis, some in pots, some in the garden. A couple of group 3 but mostly group 2. Once they have finished flowering, I leave them overwinter as they are. OK, they look dead and straggly but then it is easy to see the new buds which start to develop in spring. Late Feb early March, prune them back lightly. Find a nice fat pair of buds and cut them to just above that bud. It is only group 3 clematis which require harder pruning. You need to feed them when they are growing with Tomorite or Phostrogen to encourage flowering, until they bud and flower, then stop feeding. In Autumn, a good mulch of well rotted manure or general fertiliser to the ground around the root. Mine seem to thrive on this treatment and all have lots of fat flower buds ready to pop open even as I type. I also agree with Albags that it probably needs more light. Good luck, they are lovely climbers