My mother-in-law had the same problem. After talking with her, I learned that she was cutting the vine back to the ground every winter because the stems were so thin and brittle that she thought they were dead. Her clematis was the kind that blooms on the previous year's vines, so she was effectively removing all of the blooming wood each year. Anyway, if this isn't your problem, try adding garden lime to the soil around the base of your plant once a year. Clematis prefer alkaline soil, and the lime will help push your soil's pH in that direction. An annual application of a slow-release granular fertilizer is a good idea, too. Look for one with a high phosphorous content; something like 10-30-10, to promote blooms.