Tangerine bitter & fruit dropping too soon

Hidee frens and naybers!
I have a 6 year old tangerine tree giving fruit for past 3 years. Have been fertilizing and watering per nursery advice except no ammonium sulfate added until last year (this year adding 1 cup in spring, summer and fall per nursery advice). Fruit is falling off again this year, as in past 2 years - just after starting to turn color, about 2-inches diameter. That fruit is super bitter. Throwing it in the compost pile by the bushel this year.
It gets full sun from morning until about 3pm when shade from a nearby tree shades it for the rest of the day. Leaves, etc, all look healthy & no insects or diseased look. Soil test shows in the proper ranges.
I'm about out of ideas. Any idears about what to do try?
Bob
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Any ideas?
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