We need a lot more info than "under-developed fruit".

Pictures?

Where are you?

Have you check checked the soil/fertilizer/pH conditions?

Have you checked for pests?

As I continue to experience, the age/maturity of the tree plays a huge part in whether or not it will develop fruit.

Thank my friends for your quick replay......

i am in san bernardino, ca zone 9 and
i never check the soil/pH conditions?

what do you mean by (...pests....)

iam sorry, i am really just a new guy in gardenning , i bought alot of fruit trees but i dont know how to take care or maintain....

pls i need some advise , guide or instructions

How to check PH soil , do i have to buy something from home depot?
the fertelizer.... I bought it from home depot DYNAMITE RED COLOR the home depot guy said that the best for fruit tree......its very expensive $ 8.00 for small container........

this is the picture, finally i can download it.

all the flowers gone and now the tiny fruit also almost gone.... all dry...... i dont know why ???

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Hi. I used to live in Riverside County. I know the soil there (and probably in San Bernardino) is extremely heavy clay...that drains realllyyy slowly. Did you amend the hole when you planted it with potting soil, pearlite, etc? That might be a reason. I also know San Bernardino gets dry and hot in the summertime, so you may need to water more.

I live across the LA mountains in the Mojave desert, and I'm dead sure you have a climactic problem.

Soil pH is acidity. Examples

Some plants like acid. Some do not.

Here's a SITE that tells you everything you need to know about longans.

San Bernardino is much drier than the Southeast Asia longans originate from.

A tree that small will never hold all of it's flowers. They will turn brown and drop. If the flowers didn't get pollinated sufficiently, the fruit can drop as well. The plant is still small. If you are concerned that the plant is on a decline, then cut off all of the flower panicles, even the ones with fruit. Let the plant divert it's energies back to growing/repairing instead of trying to support fruit.

You may want to get some minor elements spray and treat the plant. The web is full of info for any plant you may have obtained. Also do some searches from this forum. I'm sure someone has your plant and has experienced similar difficulties.

I use that red container dynamite, but it is a 9 month formula, applied at the end of frost and is good till the following year(once a year formula). Apply appx 4 tablespoon per 5 gallon container. How much did you put in? I don't have longans only lychees but I would imagine they require a similar watering requirements, a little less water than lychee if I'm not mistaken. I agree, don't let the tree fruit right now, it needs to concentrate on getting healthy first and new growths. Like Jay says, trim the fruit pannicles, give it water appx. 2 times a week depending on how hot it is in your area. Watch and see how fast the water drains down to the roots. Sometimes wind damage causes leaves to turn brown. If you have it planted in a windy location. Did you also check for bugs, look under the leaves to make sure.

Good luck and hope your tree recovers...