The problem you describe on your dendrobium orchid is called bud blast. Several factors can cause it. Environmental extremes are the most common reason. Either over- or underwatering, extreme temperature fluctuations, low humidity, excess fertilizer, or low light may be at fault. Also, some insects such as aphids and thrips may cause flower buds to abort. Another possible cause is air pollution in the form of toxic fumes from paint, for example, or ethylene from ripening fruit, or even an improperly vented heater.