Clematis can be temperamental, but not that temp and after all the time they have been in situ makes me think it is the growing conditions you have that is the problem, All clematis like a deep wide hole dug where they are to grow, fill the bottom with manure or rich home made compost add some chicken pellets or other, to the compost, place the plant into this half filled hole (make sure you have planted the stem about 4 / 5 inched deeper than before, then back fill the rest of the hole with a mix of the same compost or manure mixed with garden soil, use you fist to press ? firm the soil around the root hole, make a dent around the hole and fill with water several times to soak the roots, then place a large stone or flat paver stone partly over the root hole so the root remains cool and the growing (TOP) part of the plant gets lots of sun, I would suggest that growing ANY plant against a wall requires much more water that once a week, remember the wall gets heated up by the warm sun and this in turn evaporated and draws moisture from the soil around the bottom of the wall, so plenty of water is required for plants growing in that area.
Forgot to mention that the reason for planting the Clematis stem extra inches below the soil is, these plants for some reason can pick up a disease called clematis wilt, it kills the plants if affected, and there is no cure, however, growers have discovered that Planting the stem deeper helps the plant regenerate from under the soil and throws up new shoots to regrow again. dont grow other plants too deep as they will rot, Clematis are the exception. good luck, hope it wont be too long before you are enjoying next years blooms, WeeNel.
