Answer: If your tree is not flowering, it's probably a cultural or environment problem. Lack of fertilizer could contribute to the non-flowering, as could lack of sufficient light.
Some leaf drop is common on citrus at certain times of the year, especially late winter/early spring as new growth starts. Heavy leaf drop may be caused by overwatering or overfertilizing. Citrus is generally not heavily pruned but if your tree has a peculiar growth habit, you should be able to correct the abnormalities without harming the tree.
I'd prune it back now, then feed it with a fertilizer formulated for citrus trees. Because your tree is in a container and susceptible to salt burn, reduce the amount of fertilizer you use. You can either dilute the fertilizer in water and apply a little of the mixture after you've watered the plant. Plan to use it up over the entire growing season as a supplement. Or you can sprinkle a teaspoon of granulated fertilizer over the soil surface and lightly scratch it in.
Citrus trees need as much bright light as possible during the winter months. You may want to use supplemental lighting (keep it on 14-16 hours each day) to encourage flowering in your tree.
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