Christmas cactus- Why are many of the buds falling off?

Normally let top 2 in. of soil dry between watering. When flowers are budding, Only allow top 1 in. of soil to dry. Dehydration or over watering causes buds to drop. Also these plants need low to medium light.-- To encourage flowering buds, move outdoors to shaded area, return indoors before temp. drops to 45 F.