Hi Tanya!
For phals, I believe sudden buds drop are due to
1) Temperture gets too high during bud forming and flowering (I believe <76 F is best, higher than 80 F will cause bud drop almost immediately since it makes the plant thinks it's growing time, not flowering time.
2) Large fluctuation in temperature. If the difference in daytime and night time temperature is too large, the buds drop too. Phals and other orchids like them like temperatures vary in quite a narrow range. Orchids with pseudobulbs often like and tolerate a larger day/night temperature difference.
Cheers. Hoa.