Here are the explanations for some of the most common problems:
Failure to flower: Lack of light, lack of feeding, erratic watering, or low temperatures can all cause this problem. If a citrus plant is grown from pips it will often not fruit for a number of years as they have to go through a juvenile stage before flowering. An average time for trees grown from seed to fruit is seven to ten years. Commercially, citrus are propagated by budding or grafting and can flower and fruit after only after two or three years.
Flowers fall before fruit sets: Dryness at the roots and lack of air humidity can cause failure to set fruit. Flowers do not need artificial pollinating.
Yellowing of leaves: There are several possibilities. The roots could be too wet or too dry. Draughts, low temperatures, or lack of feeding will also result in yellowing leaves.
Loss of leaves: This can be caused by draughts, too low or high temperatures in winter, often coupled with too much water in winter. Citrus prefer a cool winter rest. Provide lemons with a minimum winter night temperature of not less than 10°C (50°F) and calamondin oranges with 13°C (55°F).
Fruit fall: Fruits should ripen in a period of warm sunny weather, taking almost a year to develop to full size. Most cultivars set too much fruit for the size of the plant. Some of these will be shed, or clusters should be thinned to one fruit each on young plants.
Rotting roots: The first sign may be leaf fall or yellowing often caused by overwatering. Cut away damaged roots with some of the compost and repot into a smaller container.