I finally figured out what the trigger is thanks to a cutting on a window
sill. It's the cold. Not light. Not fertilizer. COLD.

My little cutting bloomed on a cold window sill while my big plant (30
years old) was being carefully lighted for only 8 hours a day, plunged into
absolute darkness as well as being carefully NOT watered, all without a
single bud in sight.

I dragged the big plant to the cellar -- and, believe me, that took some
work -- and put it on a cold windowsill for three weeks and BINGO! Buds.