There are different types of clematis, and they should be cut back at different times of year. Maybe you are cutting it back at the wrong time.

Yes that's the problem. I have grown two Clematis on the same arch and after a year I realised one needs to be cut back in the Autumn and one doesn't. I am going to wait until this Autumn and dig up the one that needs cutting back and place it elsewhere and replace it with Clematis that doesn't need to be cut back. I haven't been well enough to do it this time but I am definitely going to get it sorted this Autumn. It is causing problems trying to keep the two apart and identified.

I thought that summer flowering ones were to be cut back in March? perhaps not...