Hey Trung, your tree appears to be lacking Iron and Magnesium. I would also guess that the pH of your soil is around 8 or so and the pH of your water is probably around 8.2-8.4. This high pH makes it difficult for your plants to uptake certain nutrients. the Chlorophyll molecule is similar to human Hemoglobin molecule except that chlorophyll has a Mg in the center of the molecule instead of an Iron. The yellow leaves can be an indication of lack of Mg, Iron or Nitrogen or also very likely a lack of all three. I say this because these three elements are relatively mobile especially if you are watering often.
Without a soil analysis, it is difficult to diagnose the exact problem but you can always use a foliar spray to correct nutritional deficiencies without the danger of nutrient lock which can be caused by blindly applying multiple types of fertilizers.
My Longan plant had leaves that looked like yours before and I did a soil drench of chelated Iron and I also foliar sprayed with Southern Ag nutritional spray. I performed the foliar spray every week in the evening for 3-4 weeks and it corrected the problem.
If you do have high pH soil and or water, you may want to occasionally drench the root zone with low pH buffered water so that when you do fertilize, your trees can absorb more of the nutrients.
A Longan tree a couple blocks from me is about 15 feet tall and flowers every year and I e only seen it with very few fruit one time in the last three years. My small Longan was planted as a two foot stick about three years ago and it fruited every year including the first year it was planted. I believe it was due to proper nutrition and frequent watering while holding fruit. Remember that in general, if you water frequently, there is a possibility that you are watering away your fertilizer in which case you may want to apply less concentrated fertilizer at more frequent intervals.
Simon
Simon