Certified GKH Gardening Expert
Without seeing the damage, I will only be able to give you common causes for the dropping, and malformation of buds. This will help you to narrow down common causes and diagnose your plant.
This can be caused by improper fertilization, first. When the soil, either, lacks phosphorus and potassium or has an excess of nitrogen, it can cause these symptoms.
Alternatively, if it has been excessively wet in your area, it could be a fungal infection. This can be prevented and treated using fungicides. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
This article will offer more information on why Clematis will fail to flower: https://www.gardeningknowhow.com/ornamental/vines/clematis/getting-clematis-to-flower.htm