Maybe the lower parts of the vine are too shaded? Are they growing on a north-facing wall.

did you prune them this year? if not the new growth is on top of the old growth and therefore 'at the top'.

Too much nitrogen based fertilizer will promote vegetative growth at the expense of flowers. Try fertilizer specifically formulated for clematis. No fertilizer is better than the wrong type.

Yes, SB, I did prune them as I do each year but I’ve noticed that most of the plants in my garden are much taller than usual this year. Normally, the clems flower well so I must assume that I made the Miracle Grow too strong. I can be rather slapdash when feeding as the measurements on the watering can we have are on the outside in exactly the same colour as the can itself. Very difficult for me to see.
Oh well, you live and learn.
Thank you for all the advice.