Another cause of failure to bloom is too little light during the short days once buds have set. Christmas cactuses require well-drained, aerated, organic soil and bright light. Water when the top of the soil is dry. A balanced fertilizer is adequate. Stop fertilizing about a month before buds set.
Bud drop is an occasional problem.
It can be caused by temperatures above 90 or poor light after buds set. If there's not enough light during the short days, the buds can't develop. Ethylene gas, produced when there's poor ventilation near a heater or fireplace, also causes bud drop. Mail garden questions to Kathy Huber, Garden Editor, Houston Chronicle, P.O. Box 4260, Houston, Texas 77210. The bulk of mail prevents individual replies to all unpublished letters.