Sacha wrote:

I think you said that you're growing the clematis in pots with bamboos,
didn't you? Could this be the problem? Clematis like a cool root run so
they may be getting too hot and they like to be kept pretty damp - though
not water logged - at the roots. Perhaps they have a conflict of
nutritional interests with the bamboos, too?

He's got bamboo canes, not bamboo plants with them! At least that's
what I understood.

Sounds so strange - I can't think of anything beside the possibility
that the clematis starts fine because their roots are still in the pot
compost around the root ball when he plant them and as soon as the
roots get outside the compost and hit whatever soil there is they get
sick. Water logged came to mind too.