Fruit Tree Mysteries

We have some interesting, and sometimes frustrating, fruit tree mysteries. For example, we planted a dwarf julie mango that is doing very well about four years ago. We get two harvests per year. Then we planted two other dwarf julie mango trees, about 20 feet or so away from the first julie that is doing so well, and they just sort of 'sat there' for a year and a half doing next to nothing. I know I purchased the second two julies from a different nursery, so is that why they are just now starting to grow and flower? Is it the dirt or soil or rocks in that location? Do they need more fertilizer or water? It's a mystery! Anyone else have fruit tree mysteries to share?