You may want to get a soil test. Clematis thrive in soil with a pH of 6.0 to 7.0. Once you know your soil's pH, you can adjust the level with soil amendments, if necessary. You can add limestone to raise the pH and sulfur to lower it. If you have added lime to your soil in the past, adding lime to soil with a pH greater than 6.0 can lead to iron chlorosis. The symptom of iron chlorosis is yellowing between leaf veins. If your Clematis is close to a stucco or masonry wall, it can leach lime into the soil. Yellowing leaves can also indicate a lack of magnesium in the soil. My suggestion is to get your soil tested to see if you need to amend it to correct the problem.