Hi Taz, I'm wondering where you have positioned your pots, clematis love their feet in the shade and faces in the sun. If you've had a few hot days, the pots would get very warm too, these wouldn't be the best conditions for growing clematis. If I were you, I'd plant them in the garden and grow them up trellis or trees.......

This can happen for several reasons, one being damage have the stems been broken by contact with you or and animal, bird even?

What variety of clematis are they?

thanks everyone,I don,t know the name ,it,s a white one ,but my princess diane and another pink one are same, all in pots the worst one is the white and that,s on the south facing wall, I protect the roots from the sun,this year it grew really strong in spring, lots of shots,than seems to shrivel as the summer ( ha ha) progresses. could it be getting too much heat from the wall when the sun it at,s strongest. I don,t really have any spaces left in the garden as I have a cold frame full of cuttings (penstemons) and seedlings (foxgloves)to plant out next year

Perhaps its Clematis wilt? if so cut the affected stems to the ground and they should recover. Most do like their roots in shade and their heads in the sun could you stand something in front of the pots like another plant to shade their roots? Also are they in deep pots as they need to be planted deep?

I'm wondering what pruning you do - the Princess Diana one is a Prune Group 3, which means you should cut it down to about 2 buds from the soil in midwinter, or by mid February, each year. It may be that your pots are not deep/big enough too - a chimney pot particularly is very narrow, so that'd be fine if its open at the bottom and is on soil, because the clematis can push roots down into the soil, but if the pot is on a hard surface, the roots are very restricted. You can grow clematis reasonably well in pots, but the pots do need to be 2 feet deep and 12 inches wide as a minimum. They will also need feeding in pots, together with regular and sufficient watering when they're in full leaf and growing strongly.

thanks everyone, my chimney pot is three feet high and about 10 inches across , because it is facing south I have put a slate over the top to shade it,s roots from the sun, I think I will pot the others up into bigger pots in the autumn