clematis nelliemoser

If the flowers are otherwise normal then it is a problem with a lack of light and it will sort itself out when the weather improves.
If the flowers are distorted and more like leaves than flowers this may be Green leaf disease which is actually a sort of Virus infection spread by aphids and leaf hoppers. There is no cure. Remove all affected flowers and burn, If it is all flowers then dig out and burn the plant. This is not very common though, so do not be too hasty!