Lack of humidity can cause buds to dry out and drop. Plants need at least 40 to 60 percent humidity to grow and flower. Besides using a humidifier, you can place a drainage tray underneath the plant container to catch excess water and use it to provide the necessary humidity.
Moving plants away from heating and cooling vents and not exposing them to drafts from opening doors will help keep the buds healthy and fertile. Ideal temperatures for orchids are between 50 and 90 degrees Fahrenheit. Don't place the orchid where the temperature changes drastically throughout the day and night. If running an air conditioner, keep the room temperature at one setting.
Buds will dry out if the orchid soil dries out too much between waterings. The stress on the plant will cause the buds to dry out and drop. Some orchids need the soil to dry between waterings, but most orchids need water when the soil is slightly damp. During the winter, orchids need a reduced watering schedule because the plant is not actively growing. Give the orchid the correct amount of water according to the orchid variety you have growing.
Thrips and aphids will feed on flowers and buds, causing the flowers to die and the buds to turn brown and drop. Inspect plants weekly for pests and use the correct insecticide to get rid of them. For thrip control, use a product with diazinon, and for aphids, use an insecticidal spray to remove the aphids and the ants that cause the aphids.