I don't really know what's the secret.
Sometimes I totally forgotten to water it for few days and like right now, it rains for a couple of days non-stop. I think it's the drying period in between watering that makes it grow tuber. That means you let the media dry out completely before the next watering. This is base on my observation....I might be wrong.
Maybe you can try out to see how's the result.
Btw, only 1 plant left. The other plantlet just 'disappeared' after repotting.