A number of factors can cause blooming citrus not to bear fruit. Indoors, a common reason is that the blooms don't get pollinated. Use a paintbrush or cotton swab to transfer pollen to the center of the flower early in the day. If all the flowers that bloom were to set and become fruits, the plant would break down from the weight of the crop. As few as 5 percent of the flowers need to set to produce a full crop. So you need not hand-pollinate every blossom to get a decent crop. Moisture stress (too much or too little) and nutrient deficiencies can cause fruit drop too. These problems are usually associated with leaf yellowing. Keep plants uniformly moist but not wet, and use an acid-based fertilizer formulated for citrus to prevent these environmental stresses. Low humidity in the home could also cause developing fruits to abort. Make certain that your plant has adequate humidity by grouping it with other plants or using a room humidifier.