Posted: 16/06/2017 at 13:58
I'd have potted that on instead of putting it into a big container with a load of other plants right away. It's a very young plant, and it may have been grown in protected conditions and is unable to cope with it's new surroundings.
It could also be slug damage as they're quite vulnerable to that when young, or when new growth starts. I also wonder if there's too much nutrition and moisture in the soil mix? If the growing medium is too rich at that early stage, you get weaker growth which the root system can't support as it's too immature. Also - make sure you haven't got wet soil and mulch right up against those slim stems, as that can cause a bit of rot.
Take a look at the stems low down and see if they're broken. If they are, cut the stems back to reasonable joint and keep an eye out for slugs - remove them each night or use a barrier method to control them.
Did you plant it deeper than it was in the pot? That's beneficial to enable plenty of root and stem growth.
That's a few suggestions to be going on with!