It most likely is something about the soil and what the plant isn't getting, hard to say without a soil test. I did come across an old post where someone else mentioned the same clematis having the same yellow leaves issue. Several mentioned to try adding the Epsom salts as Sheri mentioned. Make sure to add lots of compost and mulch well too every year. The mulch and compost will break down to feed your soil.
Found some good care tips on Silver Star Vinery site, see Additional Tips for the yellow leaves/epsom salts formula:
http://www.silverstarvinery.co...
