This is usually caused by uneven moisture in the soil. First, it is too dry; then, it is too wet.
In the ground, you should allow the soil to dry to a depth of about 1-2 inches and then irrigate. When you irrigate, do not allow the soil to become soggy. Citrus requires good drainage.
I have three dwarf citrus in large pots. See . In the pots, I used my own home-made potting mix; see . With the current drought, these get watered 15 minutes twice each week via a drip system; previously, I watered them every third day for about 20-25 minutes.
I also have a dwarf tangelo in a raised bed framed with lose concrete blocks. For this, I dug to a depth equal to the height of the blocks. I stirred compost, wood chips, peat moss, coarse sand, and gypsum into the native clay soil. Every other year, I apply more gypsum to maintain good drainage.
Always happy to help, and if it is something I do not know myself (that covers a *LOT* of stuff!), my instinct is to find an extension service or university ag department in my immediate area that is familiar with local plantings and problems.
These sources are generally sources of excellent diagnostics and proven remedies. The only drawbacks are that often Big Ag is supported answers may be geared to professional growers, rather than home gardeners.
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