In order to flower well, longan usually needs a “winter” season with min temps <12°C and/or less rain. While fruiting, hot and wet is best. At sea level in the tropics, the low temps usually don’t occur, and even if the winter is dry enough for longan to flower (but not dry enough to kill it), the other half of the year usually doesn’t get as hot as subtropical summers, so the fruits might not develop properly. Either you have a strange tropical breed of longan, or you are very lucky to have the right conditions where you live.