Have had a couple of these plants for 6 months now and they have started to develop brown patches on the underside of their leaves. They start small and slowly spread, with the upper side of the leaf loosing colour too.
Eventually the leaf dies and drops off. There were also some tiny black specs on the underside too, but were easily removed by wiping.
Have tried the anti-fungal option and have sprayed twice with a solution of sodium bicarb and hand soap. Also used Bug-clear. Being careful with the watering. The new leaves come through fine, but before long they succumb. Any ideas?