I have a three-year-old Santa Rosa that's about 12' tall, 8" in diameter, and has produced fruit the last two years. I fertilize my trees quite heavily and irrigate. It actually bloomed its first spring, but I don't let them fruit the first year; that's for putting down roots.
Sometimes a tree just isn't healthy, or may not be in the right spot. If you planted that where an old tree was, without digging out the roots, they may be interfering. I had an Ozark Premier that I just cut down that was about 6 years old, never got more than head high, and had probably a dozen fruit its best year. Never did figure out its problem. Now it is helping flavor smoked meat and is doing okay at that.
Plums like at least a foot of soil over clay, so that there's drainage. Most fruit trees don't like to have standing water around their roots. Try another SR in a different place? Dig up the rootball and move that one?
I've kept a small orchard, about a dozen trees, mixed varieties of plums, peaches, and apples, for about 25 years. They don't last forever, even with regular spraying and maintenance. I plant new ones every 2-3 years, so that when an old one dies, a replacement is at hand...they're good to have but can be really frustrating.
