I have a fair number of young fruit trees (2-3 years old) that seem to grow well, but drop their blossoms. Is that normal? They are still young, so I wouldn't want to let them have loads of fruit, but I just wanted to make sure I'm not doing something wrong.

My avocado, pomelo, and starfruit trees all flowered this spring, but no fruit set.

For comparison, my Glenn mango (same age as the other trees) sets close to 100 fruit, and I have to pull a bunch of them off.

Although admittedly I also have a lychee tree of the same age that grows like crazy, but has yet to flower at all.