Q.Why Is My Bougainvillea Not Growing?
I bought this bougainvillea 1 year ago and it was growing lushly. Last summer I left it in the pot and all was fine, with healthy leaves growing. I transplanted it last September to the large ceramic container and since then it barely grows and the leaves are tiny. It’s now full summer where I live in Spain. The plant receives lots of sun (6 hours), I water it when the soil’s dry, I add nutrients but the leaves won’t grow. By comparison I have another bougainvillea which I transplanted around the same time to a smaller ceramic pot and this is doing fine. I’ll attach photos of the problem one and the healthy one. Any thoughts what could be the cause of the lack of growth? Any help appreciated.

Certified GKH Gardening Expert
The pot size is probably too large; they prefer to be somewhat rootbound. Plus, the larger amount of soil holds more water that can lead to root rot.
Also, the other plant in there might be grabbing all the nutrients and water! Your healthy one is in a smaller pot with no other plant.
Here is care information for growing in containers:
https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/bougainvillea-in-containers.htm