Do you know the name of your clematis. Check the plant data base here to know what type and name.
Did you prune your clematis last season. If so, it's possible you removed stems with buds that would have produced this year's blooms.
It hard to tell on some clematis, there are 3 types of clematis. Know which type you have, flowers that form on old growth and new growth.
It could also be PH or a nutrient imbalance. There are ph testers available.
If you fertilize use a low-nitrogen, steady, slow-release, organic fertilizer throughout the growing season.
Another possible problem is run-off from nearby fertilizer. Did anyone use synthetic fertilizer on the lawn or other plants, or does your neighbor's yard get rain that runs into yours?
