I have a plum tree, about 17 years old, and it's only 3 feet tall. It flowered two years ago for the first time, but didn't fruit, and hasn't flowered again. It was originally transplanted from a pot. As far as I'm concerned, it can be small if it wants to, but I've been waiting a long time for plums! Is there anything I can do? It's in a place with morning and midday sun (when it's sunny, which isn't often), with rich, well-draining soil.