Posted: 10/06/2013 at 09:06
Does the big tub have good drainage? Clematis love soft rich soil, but don't like to be waterlogged.
Cover the surface of the pot with gravel or something else to shade the root-ball.
It could also be the shock of planting, as gentle as you might have been, sometimes a plant just doesn't like to be handled.
At the worst, it could be Clematis wilt. Certain varieties are more prone to this disease (big flowering type 2) which can occur on even the healthiest looking plants.
All the top foliage will just wilt and die, but so long as the Clematis has buds beneath the ground, (which is why Clematis are planted deeper than their previous pot) it will re-shoot at first opportunity. This could be the following spring, so don't chuck it out, just leave it to recover.