Posted: 12/05/2014 at 19:59
Are you renewing some of the compost each year Vith? When they're in pots they're dependant on you for their nutrition so feeding and watering is very important but the pot grown one could simply be exhausted. Check that it's not waterlogged anyway - and put it in a bigger pot if it's a bit congested.The site is probably less of an issue but if you can move it to a more sheltered spot it may help.
It's easy to overwater plants and it causes more issues than cold, but if the C de B in the ground has decent, free draining soil, there isn't a lot of other plants in competition for nutrients,you're pruning correctly and feeding it well, there's no obvious reason for any problems. Most clematis will also have a few marked or dark leaves now and again. Can you post a few pix to give us all a better idea?