No fruit set on Santa Rosa plum

My four-year old plum ought to have given me some fruit this year. I pruned correctly, leaving plenty of last year's wood, and there were a lot of blossoms. The native bees were out and I did a bit of pollinating myself. Not one fruit set.

The tree gets adequate water and occasional fertilizer. It appears quite vigorous.

What are possible causes of this?

Even though they're self fertile, I think they produce better with a cross pollinator. That may be one issue. Otherwise, the tree is just young. I wouldn't be surprised if it does much better next year. I have a plum tree (different variety) that bloomed a lot when it was young, but produced only 1 plum last year (its 4th). This year, I got about 75 plums. Next year should be great.

My plum trees tend to get hit a lot with late frosts or freezes,  which can kill most if not all of the flowers or small fruit forming and cause them not to produce.

They'll have tons of flowers one week, and then a week later, nothing.

I've been trying to find later blooming plums that will bloom after the colder weather has passed, and hope to plant the seeds to breed new plum varieties that bloom later.

For that tree, creating a warmer microclimate around it may work to help keep it from being damaged by the cold.

These threads discuss some ideas for it.

https://permies.com/t/16542/Creating-microclimate-breadfruit-tree

https://permies.com/t/17507/Building-microclimates

Hope you get some delicious plums soon!

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.

The other issues mentioned here are certainly possible too though, depending on your location. So many variables growing food!

I have a 20 year old Santa Rosa...it is my favorite plum.  I do have a cross pollinator.  I find that here it is not reallyreliable as a producer.  Some years nothing..this has been a wet, wet spring in Eastern Washington and it’s loaded.  

I don’t think what you are experiencing is unusual.  It really didn’t start producing for me until after the 4 th or 5 th year and then some years not so much.

It's heartening to know others are having the same experience, with positive results. Thank you all.