I see no one answered your question - your description indicates the tree is
under stress of some kind. The most common problem for homeowner
("dooryard") citrus is being kept too moist. When the tree is young and
vigorous its roots can outgrow the root rot problem but by the time it's
well established root rot takes over.
If your citrus is getting the same amount of water as your lawn, it's
getting way; too much. Also, if you have mulched around the base that has
to be removed to allow everything to dry out. Citrus shouldn't have mulch,
grass or thick weeds inside the drip line.
It may not be too late to save the tree, if you can clear out around the
base and if it's not getting the same watering schedule as your vegetable
garden or lawn, but the fruit will probably not improve for at least one
year while the tree tries to recover. Keep it well fertilized and let it
dry out thoroughly between waterings, especially at this time of year.
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 ideas about what to do try?