care of begonias

they usually fall off when they still look alright with these, but should remain in situ for 3/5 days before dropping, and be readily replaced with masses more. Are you noticing them dropping because they're falling on the ground, or because the plant never has any open flowers for long?

Thanks Bamboo, have had begonias for years and know flowers fall off before dying, but these are falling off after only a day of opening. Maybe just the very hot weather, but am keeping them well watered.