So it's just staying dry to long between waterings to absorb enough K? That's a very interesting problem. Have you tried repotting it into something with a wetting agent/moisture control that dries out slower, or maybe covering the soil surface with a cheap heavy mulch to reduce evaporation?
I'm not sure what soil medium I was using at the time but a few years ago I remember when gardening my containers would dry out before I got home that evening if soaked with water that morning. I know how annoying that can be
