Flowering plants whose buds do not open, or open deformed, are usually infested with a very small insect called a thrip. You can check for this insect by taking some of those buds, cutting them open, and tapping them on a piece of paper. If thrips are present. you will see small, slender bugs crawling on the paper. They can be controlled by spraying with an insecticide labeled for indoor use and thrips. Your begonias (Begonia x hiemalis) prefer cool locations (50 to 55°F at night, 65 to 70°F during the day) with no drafts, at least four hours of curtain-filtered sunlight from a bright window, even moisture, and regular fertilization.
For more information on thrips read: Recognizing and Controlling Thrips
