Posted: 26/05/2015 at 14:06
It's not a clematis I know, but the British Clematis Society website says that it flowers at this time of year on old growth and later in the year on new growth, so it's behaving as it should, but you say the later growth is sparse. http://www.britishclematis.org.uk/plantprofiles/crystalfountain.htm
Do you feed it enough? Clematis are very hungry feeders, and this clematis produces large flowers. I would feed this type of clematis with special Clematis food every three/four weeks from March to September - hopefully this way it'll have enough energy to give you a really good display later in the year as well as early on.