It maybe the transition from pot to ground. Did you water it enough and tease the roots out a little? A friend of mine had some and they were large and full of flowers. The bad winter killed them all but they self seeded and in the same year reached two metres tall, also full of flower. I grew one last year, of a different and small variety, and it flowered very well until late September. I then cut it back and it is struggling a bit this year but it is still there and flowering. I understand that they are short lived but cuttings are easily taken. So as I say it may just be the transition from pot to ground. Try giving it a feed.

Thanks - yes, I watered it a lot but forgot to tease roots out. Shall give it a feed tomorrow. That was lucky for your friend that they self-seeded. I wasn't sure if it was a perennial or biennual?