Why doesn't a clivia bloom when its leaves and stems are growing well?

All plants only bloom and set seed when adversity arrives. This is so they can survive the adversity in the form of seeds, which are more resistant. The reason your clivia isn't blooming may be that conditions are too favorable for vegetative growth. First, clivia blooming requires short-day conditions, that is, daylight hours getting shorter day by day; the change in light gives the clivia a signal that winter is coming. Second, the day-night temperature difference needs to increase. Third, apply less nitrogen fertilizer and add a bit more phosphorus and potassium fertilizer, which can promote flower bud differentiation.