There are a few reasons when Citrus shrubs/trees have these issues. Stress to the plant like lack of water, inconsistent watering and possible nutrient deficiency. I'm assuming you are growing it in a container? A loam-based compost with good drainage. They prefer deep watering allowing the water to drain right through the pot every week over the warmer months rather than small light watering every day.
Check the leaves for any possible sucking insects, usually on the under-sides of leaves. If so, it will need cleaning and wiping off.
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Check the leaves for any possible sucking insects, usually on the under-sides of leaves. If so, it will need cleaning and wiping off.