It is usually because it is not being cut back properly. If you need advice on pruning then please let us have more details like how old it is and which clematis it is.

maybe nipping all the growing tips out will force it to bush out more and throw up more shoots but id leave it till spring .

You could try training some of the main stems horizontally.
That works for climbing roses.

Steragram is right. I have been told to train some stems horizontally on a fence or spirally round an obelisk in order to get blooms lower down. It seems to work quite well but you have to keep up with the tying in and be very gentle or the plant might decide to sulk or wilt!

it worked with my cherry tree training it and nipping the tip out as that got a bit top heavy .