I don't grow clematis but I often hear they want their feet in the shade and their head in the sun. Double check that aspect - the planting depth, good drainage and cover with a few inches of mulch to keep the roots cool, moist & shaded. Be careful with fertilizer. Too much is no good.

I've done all that Paul the one which is doing well has been planted just the same and in same aspect.

I've got one clematis on a north fence, which has been there for years and seems to do well but I have a clematis Princess Diana which has been all around the garden because it hated that aspect. I found that the stems had been rasped away by snails and it had the same problem as yours, so now it's in a pot facing east and it seems perfectly happy.