Having problems with my Promenea

[camille1585]

I have a little Noid Promenaea that I picked up for nearly nothing at a local show last October. Tag says it's a yellow hybrid, that's all. Despite me knowing nothing about this genus it's been growing quite well. Watering has been done by trial and error, I don't know how much they need! Judging by the pleating, I need to water more.

Anyway, in the past week or so several of the leaves coming off the top of the bulbs have gone yellow and brown. I don't think it's normal leaf drop, the affected leaves are on the more recent bulbs. And I'm also having trouble with the center leaf rotting on half grown growths.
Any ideas?