Same with mine this year, nothing. It bloomed amazingly but no baby fruits at all. I've read that some fruit trees produce every other year if not thinned because the tree is overworked one year and then takes a break the next. I didn't think we had too many last year but it was a decent crop and I did not thin so it's likely that's the cause. We have lots of other plum trees so it's getting cross pollinated and it was a nice mild winter and spring so no late frosts. I'm inclined to believe the every other year theory for some varieties if not hand thinned. Our old homestead plums produce every year but do their own thinning (we call it the June drop). I do know some of these newer varieties (and by new, I mean last hundred years or so) don't self thin well enough or at all. I read that was the case with apples, specifically Fuji, really needing hand thinning. This was from Michael Phillips,
who I believe to be very knowledgeable on the subject.
I'd be willing to bet we need to thin our Santa Rosas.