Digging this up again: I've observed a very similar problem.
Started with 8 B. caapi from seedlings. One grew very well to about 50cm size, the others up to 25cm height. At that point growth stopped almost completely on all of them. Leaves start to have brown spots, then holes. New leaves form but shrivel and dry out and drop when still smaller than 5mm. They survive like that for some months now, but are not growing anymore. I've potted them up, in different mixes (for example compost + coco choir). Gave them a Neem oil treatment. Tried different fertilizers and different amounts of water. Brought some of them to the bathroom for higher humidity. Still no growth.
Anyone having hints on how to make B. caapi happy? How much light, how much water, how much fertilizier? It is lack of humidity, do they need a green house?