Hi, welcome to GoY, when you planted it, were the roots going round and round in a tight ball in the pot, if so you will need to lift it carefully, and gently try and untangle some of the roots so that they get in contact with the soil.
What happens when a plant is left too long in the same pot, the roots quickly fill the pot as they search for nutrients, which makes it harder for them to take up nutrients and water, and they get used to going round in circles, and can't spread out when they're planted out, which results in either no growth at all, or very little,as the plant is so used to going round, so you have to try an seperate some of the roots, it doesn't matter if you break a few in the process, they will soon be replaced, as the plant once again starts to take up nutrients, Derek.