Hard to say, sometimes it can't set properly because tree is still too young, weather related etc.
I find on plum trees, it can still take several years before seeing any fruits, even if you see them flowering.
How old are these trees?

Thanks Konrad, the trees have been in the ground for about 3 years. I know there are various factors as to why fruit set or do not set but I wanted to see if I could specifically rule out cross pollination as a factor.

I guess the only answer I need to know is if you do see a tiny fruit develop, does that mean it was successfully cross pollinated or would tiny fruit develop anyways without proper pollination?

Thanks!

If you want to be sure, you can try pollinating it yourself. I've used paint brushes to pollinate a Seckel pear, and while not knowing for sure if it was me, I had very good fruit set (i used flowers off a local Bradford pear)... If you have the proper cross pollination and u see bees working your trees..i'd bet that if that fruit fails to develop, that it wasn't pollination and some other factor...

Brownmola

Based on my experience, what you describe can happen without pollination. The little fruitlets appear to begin growing, but soon stop, turn yellow, and fall off. Are your two trees japanese plums and are they different cultivars?

Thanks Dave. They are Japanese plums, Beauty and Satsuma.

What you described is exactly what I want to know because I also have several newly planted pluots that definitely need pollenizers. I have one Burgundy plum that I specifically planted to be the pluot pollenizer but was wondering if I should plant another to help with pollenization. I was going to see if little fruitlets formed and if they were created by pollenization then I was not going to buy another Burgundy to help pollenate.

I have exactly the same problem with my Burgundy Plum. I was told that it was self-pollinating and that I could use a soft artist's brush to pollinate it. The tiny fruit formed all over the 2-year old tree. (Bought from nursery as 3 y. o. tree with minimal fruit, so itâÂÂs 5 now.) Within a week, they newly-formed plum nubs were all gone, but I couldnâÂÂt find one stem on the ground anywhere. Last year the valley was hit by grasshoppers and I lost all new growth. What was it this year? Am I cursed or is my green thumb turning black?
PS The tree has dwarf peaches grafted to it; both seem to be flourishing, but it had only one blossom.
Thanks in advance for any help you can offer!

My newly planted burgundy plum has several decent sized fruit on it currently. My satsuma is flowering like crazy but my beauty plum only has a handful of flowers but most of the branches are still dormant. I know the satsuma needs the beauty for pollination so I have been using a brush from the 3 or 4 flowers off of the beauty. If the beauty doesn't fruit this year I may pull it which would be a shame after 3+ years in the ground. It has a 3 inch caliper trunk so I don't think it is too young. I did have the sense to try and graft some scions from the beauty onto the satsuma in case the beauty is no longer around next year...