if it has a poor root system then it will struggle to take up nutrients and so struggle to thrive. pot it in a mix of john innes and compost so it has a more gritty open soil to grow in. don't put it in a very big pot just one big enough to allow the root to grow outwards by a couple of inches. keep it in semi shade to prevent leaf scorch too.

when you get roots at the bottom of the pot repot on into a slightly bigger pot.
when you planted it out did you tease the roots out so they weren't going round and round as if still in the pot? was the hole at least half as big again as the pot it came out of and did you loosen the soil around the hole so the roots could penetrate into the soil?

sorry lots of questions. I noticed a neighbour planting a small shrub into his garden a few yrs ago. he commented that it wasn't growing. when he lifted it the hole was clay and looked as if he had just dug it. the roots had no where to grow into. after loosening the soil and mixing in some grit and compost and replanting, it has doubled in size and he is a happy chappy now.

Thanks, Seaburngirl for these words of wisdom! I can't remember actually planting it but from what you have said maybe it was the wrong soil. I am now off to pot it as your have described and hopefully it will thrive. It certainly doesn't want to die. Thanks again!